turn std::oldmap into a wrapper around LinearMap
auto merge of #5509 : thestinger/rust/oldmap, r=catamorphism

The reasoning for doing it this way is that it's much easier to transition method-by-method to the `Map` API than trying to do the migration all at once.

I found an issue unrelated to my changes in one of the run-fail tests - if it uses `LinearMap`, it still fails but exits with 0. I xfailed it for now and opened [an issue](https://github.com/mozilla/rust/issues/5512), because it's not caused by these changes.