My plans for breaking cleanups near ExprTrait

[Expurple]

As discussed here, I won't be able to release everything I want in SeaORM 1.0. So, I need to track my wishlist somewhere. I also need a URL to include in the TODOs in the code. This is the place.

Background on ExprTrait

• Unify `Expr` and `SimpleExpr`, add `ExprTrait`? #771
• Add ExprTrait (#771) #791

My plans

• Delete the original inherent methods of Expr and SimpleExpr which have been moved to ExprTrait and now simply use the implementation from the trait. This is a relatively small breaking change, because all signatures are exactly the same and the compiler will simply suggest to add a use sea_query::ExprTrait.
• Replace all impl SqliteExpr blocks with a single wider impl<T> SqliteExpr for T where T: ExprTrait {}
• Replace all impl PgExpr blocks with a single wider impl<T> PgExpr for T where T: ExprTrait {}