Fix #674: Unbox to Long does not convert null to 0L.
Simplify a bit unboxing functions.
Simplify isByte and isShort.
Remove the useless boxValue function on class data.
Review helpers for notify, notifyAll and finalize.
Use switches when doing multiple typeofs.
Remove useless calls to empty constructors in ArrayClass.
Merge pull request #675 from sjrd/cleanup-env

Cleanup scalajsenv.js