Embed FastTypeTags within SPickler and Unpickler WIP
Embed FastTypeTag also in generated Unpicklers

Also adds `def tag` to various custom picklers.
Use implicit Unpickler[Any] instead of dispatch logic
Replace unpickler dispatch logic with open sum unpickler macro

State of tests: 15 tests fail out of 293 tests.
Handle null, refs, and singletons in abstract open-sum unpickler

Also fixes a bug in the runtime unpickler where the order in which
fields are unpickled could be changed, leading to a crash in the
binary pickle reader.
Clean-ups

- Remove obsolete dispatch logic in `unpickle` macro.
- Add implicit Unpickler[T] parameter to `unpickle` extension
  method.
Compile-time error has changed
WIP and fixups
Re-enable custom error messages
Clean-ups

- Remove obsolete compile-time test from unpickle macro.
- Remove obsolete FastTypeTag context bounds from generic-spickler
  test.
Remove unneeded imports in open-sum unpickler macro
Remove commented code
Merge pull request #245 from scala/fasttags-picklers

Embed FastTypeTag within SPickler and Unpickler