Expose specialized versions of `AHasher` through `specialized-hashers` feature

[fu5ha]

This adds a specialized-hashers feature which exposes the existing specialized hasher implementations that were under the nightly-only specialization feature for manual use. To support this, also adds BuildAHasherU64, BuildAHasherFixed, and BuildAHasherStr types that can be used in places that expect BuildHashers and which leverage existing infrastructure to create these specialized versions of hashers.