Remove usage of @ast::Crate in favor of ownership

This patch exposes actual ownership of an `ast::Crate` structure so it's not implicitly copied and reference counted via `@`.

The main purpose for this patch was to get rid of the massive spike in memory during the start of the compiler (this can be seen on isrustfastyet). The reason that this spike exists is that during `phase_2` we're creating many copies of the crate by folding. Because these are reference counted, all instances of the old crates aren't dropped until the end of the function, which is why so much memory is accumulated.

This patch exposes true ownership of the crate, meaning that it will be destroyed ASAP when requested. There are no code changes except for dealing with actual ownership of the crate. The large spike is then avoided: http://i.imgur.com/IO3NENy.png

This shouldn't help our overall memory usage (that still is the highest at the end), but if we ever manage to bring that down it should help us not have a 1GB spike at the beginning of compilation.