Replace uses of reinterpret_cast with transmute, remove reinterpret_cast

[brson]

reinterpret_cast is rarely needed now, maybe never. Try to replace all uses with transmute, then remove reinterpret_cast.

[catamorphism]

Using some of my bug janitor time to work on this.

[alexcrichton]

This was mostly done in #5979, and the snapshots of 8081e8d fully removed this function form cast.rs