Less bad copy throughout libsyntax/librustc

Removes a lot of instances of `/*bad*/ copy` throughout libsyntax/librustc. On the plus side, this shaves about 2s off of the runtime when compiling `librustc` with optimizations.

Ideally I would have run a profiler to figure out which copies are the most critical to remove, but in reality there was a liberal amount of `git grep`s along with some spot checking and removing the easy ones.