The `impl_bytelike_traits` macro is public and it is used in the
`hash_newtype` macro, also public.
Currently if a user calls the `hash_newtype` macro in a crate that
depends on `hashes` without the `serde` feature enabled and with no
`serde` dependency everything works. However if the user then adds a
dependency that happens to enable the `serde` feature in `hashes` their
build will blow up because `serde` code will start getting called from
the original crate's call to `hash_newtype`.
Pull the serde stuff out of `hash_newtype` and provide a macro to
implement it `impl_serde_for_newtype`.
766f498b33