doc: use backticks for code-in-comments.
auto merge of #11996 : lecram/rust/master, r=pcwalton

This Pull Request aims to add backticks to all instances of code inside comments in snippets present in tutorial.md. It also includes backticks for filenames in the same conditions. See #11796 for motivation.
pull extra::{serialize, ebml} into a separate libserialize crate

- `extra::json` didn't make the cut, because of `extra::json` required
   dep on `extra::TreeMap`. If/when `extra::TreeMap` moves out of `extra`,
   then `extra::json` could move into `serialize`
- `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
extra: inline extra::serialize stub
fix stuff clobbered in rebase + don't reexport serialize::serialize
no more globs in libserialize