Pre-release: Shallow and Optional Types

[ben_greenman]

@rscho

Does this mean that libs heavily using typed/racket, such as math/array might become usable from plain racket by just changing the #lang line at the top of every lib file? @soegaard, could we finally be removing the 50x performance penalty warning soon?

I hope so! Let's talk more at racket/math #75

@MichaelBurge

Would it be possible to apply optimizations by assuming the type signatures are correct?

For extreme speed, I'd go with unsafe-provide and unsafe-require/typed
(in #lang typed/racket)