Beef up the build system.

- Added several missing .mz files.
- Added the corresponding builtin functions in MezzoLib (these currently fail,
  but at least they're here).
- Updated the .mllib files.
- Fixed a few bugs in the compiler (translation to OCaml).
- There's now one big OCamlbuild command that:
    * builds Mezzo,
    * builds the Mezzo runtime library,
    * builds the Mezzo core library using the freshly-built Mezzo compiler,
    * builds the Mezzo standard library using the freshly-built Mezzo compiler,
    * packages the three items above as separate libraries.

The details involve quite a few gory OCamlbuild custom plugins.