Remove the magic deref behavior of newtype structs and enums

This removes the feature where newtype structs can be dereferenced like pointers, and likewise where certain enums can be dereferenced (which I imagine nobody realized still existed). This ad-hoc behavior is to be replaced by a more general overloadable dereference trait in the future.

I've been nursing this patch for two months and think it's about rebased up to master.

@nikomatsakis this makes a bunch of your type checking code noticeably uglier.