Replace mentions of return with pure in the compiler

With the existence of -Wnoncanonical-monad-instances in -Wcompat and plans to deprecate return at some point in the future, I would expect to be moving towards eliminating return from compiler internals and replacing its uses with pure.

It's not as straightforward as using a regex-replace s/return/pure because of comments, pragmas and strings. I'm also not sure if this would introduce breakages, but I hope not; If so, the tests should catch that.