Replaced many instances of reinterpret_cast with transmute
auto merge of #5979 : Thiez/rust/no_reinterpret_cast, r=catamorphism

As the name suggests this replaces many instances of cast::reinterpret_cast by cast::transmute. It's essentially the boring part of fixing #5163, the remaining reinterpret_casts should be more tricky to remove (unless I missed a boring case).

r? @catamorphism