Remove ptr-int transmute by converting UnsafeWeakPointer from usize to *mut T

[saethlin]

This transmute from a Weak<T> to a usize is shockingly common in both the ecosystem and the standard library. It's doubly bad because Weak<T> is a repr(Rust) type, so without manually inspecting its layout at runtime it is a mistake to assume it has any particular layout. But primarily, this is a round-trip of a pointer through an integer type via transmute, and it's pretty clear Rust cannot have a sound memory model that permits these alongside the usual suite of compiler optimizations (casts are different, they're annoying but possible to support).

If you're looking for other examples of this approach, rayon does a very similar thing to get pointers between threads: https://github.com/rayon-rs/rayon/blob/5b6adbbf5c6944ec42b54530820ee8c0ad0e35a9/src/lib.rs#L123-L147

[tatsuya6502]

Thank you for reviewing and fixing the unsafe codes. Greatly appreciated!

I left a comment about reverting the crossbeam-epoch version. All other parts look great to me.

[tatsuya6502]

Thanks. It looks good to me!