chore: improve rustc-hash maintenance path

[vip892766gma]

Summary:

• Add unit tests covering edge cases of FxHasher: hashing of empty byte slices, single-byte inputs, mixed write_u8/write_u32/write_u64 sequences producing distinct hashes from write(&[..]), and verifying FxBuildHasher::default() and FxHashMap basic insert/get round-trip. Also add a small doctest or #[test] asserting that hashing the same bytes via different write_* paths is deterministic across calls.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[bjorn3]

Some of these tests are for properties that are handled by the standard library rather than rustc-hashes, some other tests can't actually fail unless you intentionally try to cause them to fail when making. None of them seem actually useful to me.

[apiraino]

this user has been suspended, rust-lang/rust#156958 (comment)