Make sync::raw::Sem use Unsafe to manage mutability instead of transmuting an unsafe pointer
auto merge of #14708 : gereeter/rust/faster-sem, r=alexcrichton

Currently, `Sem`, which is used as a building block for all the blocking primitives, uses a very ugly hack to implement `Share` and be able to mutate the stored `WaitQueue` by hiding it all behind a `transmute`d `*()`. This PR replaces all that ugly machinery with `Unsafe`. Beyond being cleaner and not requiring `transmute`, this removes an allocation in the creation and removes an indirection for access.