This fixes bug #68.
Previously, we had used rewrite rules, which was unreliable: they would only fire if a client library was compiled with optimisation, an unsafe assumption to make.
This has negligible effect on performance, but should be more secure than the chained Wang hash we had been using.
--HG-- rename : Data/Hashable.hs => Data/Hashable/Class.hs
Now hashWithSalt is the only method of the class. This reduces implementor confusion, and ensures that the salt should always be used.
These are *really* fast: 1.5ns for the 32-bit hash, and 2ns for the 64-bit.