pull extra::{serialize, ebml} into a separate libserialize crate

- `extra::json` didn't make the cut, because of `extra::json`'s required
   dep on `extra::TreeMap`. If/when `extra::TreeMap` moves out of `extra`,
   then `extra::json` could move into `libserialize`
- `libextra`, `libsyntax` and `librustc` depend on the newly created
  `libserialize`
- The extensions to various `extra` types like `DList`, `RingBuf`, `TreeMap`
  and `TreeSet` for `Encodable`/`Decodable` were moved into the respective
  modules in `extra`
- There is some trickery, evident in `src/libextra/lib.rs` where a stub
  of `extra::serialize` is set up (in `src/libextra/serialize.rs`) for
  use in the stage0 build, where the snapshot rustc is still making
  deriving for `Encodable` and `Decodable` point at extra. Big props to
  @huonw for help working out the re-export solution for this
- @pcwalton's change in 449a7a8 didn't sneak back in