Add uninit intrinsic
Replace init() with uninit() where appropriate
Make staged versions of the functions that use uninit
Add intrinsic declaration where I missed them
auto merge of #6354 : Aatch/rust/uninit-intrinsic, r=graydon

Adds an `uninit` intrinsic.

It's just an empty function, so llvm optimizes it down to nothing.

I changed all of the `init` intrinsic usages to `uninit` where it seemed appropriate to.