Dogfood TransmuteFrom in the Rust standard library

[jswrenn]

There are many instances of transmutation in the compiler, both used as a documented type-level bound (e.g., "this associated type must be transmutable into Self) and as a concrete operation. We should dogfood TransmuteFrom by replacing as many instances of documented transmutation bounds with TransmuteFrom and concrete transmutes with TransmuteFrom::transmute. This both will reduce the amount of unsafe in the Rust standard library and make unavoidable unsafe a little less dangerous.

Blockers:

• Until Implement intrinsic matching semantics of TransmuteFrom::transmute #18 is resolved, replacing mem::transmute with TransmuteFrom::transmute might regress codegen/performance
• At a type level, there are still rough edges with TransmuteFrom. It's slow to analyze very large types, and does not yet normalize associated types on traits (and thus cannot analyze them).