Remove ty_nil and replace with zero-length tuple

Distinguishing zero-arity tuples as `nil` causes annoying discontinuities in the compiler. We should remove `ty_nil` and just use `ty_tup` with a zero-length list of types. I [tried to do this](https://github.com/nikomatsakis/rust/tree/remove-unit), though, and encountered errors in the exhaustiveness checker that I did not understand. 
