std: minor whitespace cleanup
replace manual Hash impls with `#[deriving(Hash)]`
test: Use `#[deriving(Hash)]` in a couple tests
rustc: rename `hash` to the proper name `state`
auto merge of #12522 : erickt/rust/hash, r=alexcrichton

This patch series does a couple things:

* replaces manual `Hash` implementations with `#[deriving(Hash)]`
* adds `Hash` back to `std::prelude`
* minor cleanup of whitespace and variable names.