Implemented BuildHasher for WyHash

[LucaCappelletti94]

As per the title, added a new struct which implements the BuildHasher trait and relative test suite.

[eldruin]

Thank you for your proposal.
However, I do not understand the benefit of these methods.
IIUC, the structs can already be created with the methods default(), with_seed() and new(). Which offer the same possibilities.

[LucaCappelletti94]

Indeed, but there are crates down the dependency line that require a BuildHasher object other than the Hasher object itself. I agree that there should be only a need for the Hasher in these cases, but these crates chose to require the BuildHasher trait. If you prefer, I could wrap it under a feature flag.