Remove DVec and all uses

Closes #4985 by removing the `dvec` module and all use cases throughout the compiler.

A number of uses were directly convertible to `let mut foo = ~[];`, while others in hash maps and some fields had to be converted to `@mut ~[T]`. A small number of `DVec` instances in fields were able to be converted directly to `~[T]` without the `@`, but this was a difficult thing to do.