Make CallHasher specific to ahash::RandomState

[dtolnay]

As pointed out in #242 (comment), the CallHasher trait used by the specialization feature does not need to work for arbitrary B: BuildHasher.

aHash/src/specialize.rs

Lines 20 to 22 in 3a54154

	pub(crate) trait CallHasher {
	fn get_hash<H: Hash + ?Sized, B: BuildHasher>(value: &H, build_hasher: &B) -> u64;
	}

It's enough to work for ahash::random_state::RandomState only. This trait is private to this crate and is only ever used with ahash's RandomState.

aHash/src/specialize.rs

Lines 19 to 21 in 6dfeeae

	pub(crate) trait CallHasher {
	fn get_hash<H: Hash + ?Sized>(value: &H, random_state: &RandomState) -> u64;
	}

This simplifies the existing nightly-only specialization and also the stable version in #242 by eliminating some unreachable branches.