rustc: Use CStore, not a separate crate cache

rustc: Use CStore, not a separate crate cache

This separate crate cache is one factor which is causing libstd to be loaded
twice during normal compilation. The crates loaded for syntax extensions have a
separate cache than the crates loaded for linking, so all crates are loaded once
per #[phase] they're tagged with.

This removes the cache and instead uses the CStore structure itself as the cache
for loaded crates. This should allow crates loaded during the syntax phase to be
shared with the crates loaded during the link phase.