refactor(ast): squash nested enums (oxc-project#3115)

OK, this is a big one...

I have done this as part of work on Traversable AST, but I believe it
has wider benefits, so thought better to spin it off into its own PR.

## What this PR does

This PR squashes all nested AST enum types (oxc-project#2685).

e.g.: Previously:

```rs
pub enum Statement<'a> {
    BlockStatement(Box<'a, BlockStatement<'a>>),
    /* ...other Statement variants... */
    Declaration(Declaration<'a>),
}

pub enum Declaration<'a> {
    VariableDeclaration(Box<'a, VariableDeclaration<'a>>),
    /* ...other Declaration variants... */
}
```

After this PR:

```rs
#[repr(C, u8)]
pub enum Statement<'a> {
    BlockStatement(Box<'a, BlockStatement<'a>>) = 0,
    /* ...other Statement variants... */

    VariableDeclaration(Box<'a, VariableDeclaration<'a>>) = 32,
    /* ...other Declaration variants... */
}

#[repr(C, u8)]
pub enum Declaration<'a> {
    VariableDeclaration(Box<'a, VariableDeclaration<'a>>) = 32,
    /* ...other Declaration variants... */
}
```

All `Declaration`'s variants are combined into `Statement`, but
`Declaration` type still exists.

As both types are `#[repr(C, u8)]`, and the discriminants are aligned, a
`Declaration` can be transmuted to a `Statement` at zero cost.

This is the same thing as oxc-project#2847, but here applied to *all* nested enums
in the AST, and with improved helper methods.

No enums increase in size, and a few get smaller. Indirection is reduced
for some types (this removes multiple levels of boxing).

## Why?

1. It is a prerequisite for Traversable AST (oxc-project#2987).
2. It would help a lot with AST Transfer (oxc-project#2409) - it solves the only
remaining blocker for this.
3. It is a step closer to making the whole AST `#[repr(C)]`.

## Why is it a good thing for the AST to be `#[repr(C)]`?

Oxc's direction appears to be increasingly to build up control over the
fundamental primitives we use, in order to unlock performance and
features. We have our own allocator, our own custom implementations for
`Box` and `Vec`, our own `IndexVec` (TBC). The AST is the central
building block of Oxc, and taking control of its memory layout feels
like a step in this same direction.

Oxc has a major advantage over other similar libraries in that it keeps
all the AST data in an arena. This opens the door to treating the AST
either as Rust types or as *pure data* (just bytes). That data can be
moved around and manipulated beyond what Rust natively allows.

However, to enable that, the types need to be well-specified, with
completely stable layouts. `#[repr(C)]` is the only tool Rust provides
to do this.

Once the types are `#[repr(C)]`, various features become possible:

1. Cheap transfer of the AST across boundaries without ser/deser - the
property used by AST Transfer.
2. Having multiple versions of the AST (standard, read-only,
traversable), and these AST representations can be converted to one
other at zero cost via transmute - the property used by Traversable AST
scheme.
3. Caching AST data on disk (oxc-project#3079) or transferring across network.
4. Stuff we haven't thought of yet!

Allowing the AST to be treated as pure data will likely unlock other
"next level" features further down the track (caching for "edge
bundling" comes to mind).

## The problem with `#[repr(C)]`

It's not *required* to squash nested enums to make the AST `#[repr(C)]`.

But the problem with `#[repr(C)]` is that it disables some compiler
optimizations. Without `#[repr(C)]`, the compiler squashes enums itself
in some cases (which is how `Statement` is currently 16 bytes). But
making the types `#[repr(C)]` as they are currently disables this
optimization.

So this PR essentially makes explicit what the compiler is already doing
- and in fact goes a bit further with the optimization than the compiler
is able to, in squashing 3 or 4 layers of nested enums (the compiler
only does up to 2 layers).

## Implementation

One enum "inheriting" variants from another is implemented with
`inherit_variants!` macro.

```rs
inherit_variants! {
#[repr(C, u8)]
pub enum Statement<'a> {
    BlockStatement(Box<'a, BlockStatement<'a>>),
    /* ...other Statement variants... */
    
    // `Declaration` variants added here by `inherit_variants!` macro
    @inherit Declaration
    // `ModuleDeclaration` variants added here by `inherit_variants!` macro
    @inherit ModuleDeclaration
}
}
```

The macro is *fairly* lightweight, and I think the above is quite easy
to understand. No proc macros.

The macro also implements utility methods for converting between enums
e.g. `Statement::as_declaration`. These methods are all zero-cost
(essentially transmutes).

New patterns for dealing with nested enums are introduced:

Creation:

```rs
// Old
let stmt = Statement::Declaration(Declaration::VariableDeclaration(var_decl));

// New
let stmt = Statement::VariableDeclaration(var_decl);
```

Conversion:

```rs
// Old
let stmt = Statement::Declaration(decl);

// New
let stmt = Statement::from(decl);
```

Testing:

```rs
// Old
if matches!(stmt, Statement::Declaration(_)) { }
if matches!(stmt, Statement::ModuleDeclaration(m) if m.is_import()) { }

// New
if stmt.is_declaration() { }
if matches!(stmt, Statement::ImportDeclaration(_)) { }
```

Branching:

```rs
// Old
if let Statement::Declaration(decl) = &stmt { decl.do_stuff() };

// New
if let Some(decl) = stmt.as_declaration() { decl.do_stuff() };
```

Matching:

```rs
// Old
match stmt {
    Statement::Declaration(decl) => visitor.visit(decl),
}

// New (exhaustive match)
match stmt {
    match_declaration!(Statement) => visitor.visit(stmt.to_declaration()),
}

// New (alternative)
match stmt {
    _ if stmt.is_declaration() => visitor.visit(stmt.to_declaration()),
}
```

New syntax has pluses and minuses vs the old. `match` syntax is worse,
but when working with a deeply nested enum, the code is much nicer -
it's shorter and easier to read.

This PR removes 200 lines from the linter with changes like this:


https://github.com/oxc-project/oxc/pull/3115/files#diff-dc417ff57352da6727a760ec6dee22de6816f8231fb69dbef1bf05d478699103L92-R95

```diff
- let AssignmentTarget::SimpleAssignmentTarget(simple_assignment_target) =
-     &assignment_expr.left
- else {
-     return;
- };
- let SimpleAssignmentTarget::AssignmentTargetIdentifier(ident) =
-     simple_assignment_target
+ let AssignmentTarget::AssignmentTargetIdentifier(ident) = &assignment_expr.left
else {
    return;
};
```

crates/oxc_allocator/src/arena.rs

@@ -39,6 +39,15 @@ impl<'alloc, T> Box<'alloc, T> {
     pub fn new_in(x: T, alloc: &Allocator) -> Self {
         Self(alloc.alloc(x).into(), PhantomData)
     }
+
+    /// Create a fake `Box` with a dangling pointer.
+    /// # SAFETY
+    /// Safe to create, but must never be dereferenced, as does not point to a valid `T`.
+    /// Only purpose is for mocking types without allocating for const assertions.
+    #[allow(unsafe_code, clippy::missing_safety_doc)]
+    pub const unsafe fn dangling() -> Self {
+        Self(NonNull::dangling(), PhantomData)
+    }
 }
 
 impl<'alloc, T: ?Sized> ops::Deref for Box<'alloc, T> {