fix compilation errors if only the feature hash2curve is enabled

[alexanderkjall]

Without this patch:

$ cargo test --no-default-features --features hash2curve
   Compiling p256 v0.13.2 (/home/capitol/project/elliptic-curves/p256)
error[E0432]: unresolved import `crate::FieldElement`
  --> p256/src/arithmetic/hash2curve.rs:96:45
   |
96 |     use crate::{arithmetic::field::MODULUS, FieldElement, NistP256, Scalar, U256};
   |                                             ^^^^^^^^^^^^ no `FieldElement` in the root
   |
   = help: consider importing this struct through its public re-export instead:
           crate::arithmetic::FieldElement
note: found an item that was configured out
  --> p256/src/lib.rs:47:28
   |
47 | pub use arithmetic::field::FieldElement;
   |                            ^^^^^^^^^^^^
   = note: the item is gated behind the `expose-field` feature

error[E0432]: unresolved import `sha2`
   --> p256/src/arithmetic/hash2curve.rs:108:9
    |
108 |     use sha2::Sha256;
    |         ^^^^ use of undeclared crate or module `sha2`

error[E0432]: unresolved import `p256::ecdsa`
 --> p256/tests/ecdsa.rs:7:5
  |
7 |     ecdsa::{SigningKey, VerifyingKey},
  |     ^^^^^ could not find `ecdsa` in `p256`

error[E0283]: type annotations needed
  --> p256/tests/ecdsa.rs:14:70
   |
14 |         <NonZeroScalar as Reduce<U256>>::reduce_bytes(&bytes.into()).into()
   |                                                                      ^^^^
   |
   = note: cannot satisfy `_: From<NonZeroScalar<NistP256>>`
   = note: required for `NonZeroScalar<NistP256>` to implement `Into<_>`
help: try using a fully qualified path to specify the expected types
   |
14 |         <NonZeroScalar<NistP256> as Into<T>>::into(<NonZeroScalar as Reduce<U256>>::reduce_bytes(&bytes.into()))
   |         +++++++++++++++++++++++++++++++++++++++++++                                                            ~

error[E0282]: type annotations needed
  --> p256/tests/ecdsa.rs:22:30
   |
22 |         let (signature, v) = sk.sign_recoverable(msg).unwrap();
   |                              ^^ cannot infer type

Some errors have detailed explanations: E0282, E0283, E0432.
For more information about an error, try `rustc --explain E0282`.
error: could not compile `p256` (test "ecdsa") due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
warning: unused import: `group::cofactor::CofactorGroup`
   --> p256/src/arithmetic/hash2curve.rs:101:9
    |
101 |         group::cofactor::CofactorGroup,
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

warning: unused import: `MapToCurve`
   --> p256/src/arithmetic/hash2curve.rs:102:64
    |
102 |         hash2curve::{self, ExpandMsgXmd, FromOkm, GroupDigest, MapToCurve, OsswuMap},
    |                                                                ^^^^^^^^^^

For more information about this error, try `rustc --explain E0432`.
warning: `p256` (lib test) generated 2 warnings
error: could not compile `p256` (lib test) due to 2 previous errors; 2 warnings emitted

[tarcieri]

Suggested change

	#![cfg(feature = "expose-field")]
	#[cfg(feature = "expose-field")]

[alexanderkjall]

Fixed, that increased the diff a bit, but made it more correct I think. Thanks for the feedback

[tarcieri]

How is this related to hash2curve?

[alexanderkjall]

reworked this a bit, it was an error that showed up after I fixed the one in the hash2curve.rs file. Hopefully more correct now.

[tarcieri]

I took a slightly different approach in #998

[tarcieri]

Merged #996 instead which fixes the tests and adds them to CI

[alexanderkjall]

That was a much better approach, thanks :)