get rid of std::oldmap (migrate to core::hashmap)

[thestinger]

This is at least 30% slower than core::hashmap, not sendable, and it doesn't use explicit self or expose the underlying mutability through the API (relies on mut fields). It also results in a lot of code bloat and is probably responsible for slowing down the compiler by a significant amount.

[Thiez]

I actually have a version of librustc/libsyntax with all the oldmaps replaced by LinearMap/LinearSet, but it is blocked by what appears to be a bug in the codegen of the original compiler. When type 'map' in libsyntax/ast_map.rs is a "@ mut LinearMap" libsyntax will happily compile (assuming you make the appropriate changes in libsyntax and librustc), but when compiling the new librustc the compiler will ICE (in add_root_cleanup from librustc/middle/trans/base.rs). Sadly I have not yet managed to find the exact cause of this bug.

[nikomatsakis]

I have some suspicions as to the cause of this bug. It's related I'm sure to #3511, #3850, and the general refactoring needed about cleanups.