Use std::hash::Hash and std::hash::Hasher interfaces

[eddiew]

This enables us to write

#[deriving (Hash)]
struct X { ... }

to get hash(&self, hasher: &mut H)
instead of manually defining how to feed values into the hasher.

As part of this we'll be able to define hashes without having to go through the indirection of serialization, which is nice

I'll handle this myself over the next few days, but feedback is welcome

[eddiew]

Hasher is restricted to u64 output, so maybe we can't do this...
See rust-lang/rfcs#1768

[eddiew]

We extended Hasher to GeneralHasher to solve this.