Remove RefCell::{with, with_mut}

These are superfluous now that we have fixed rvalue lifetimes and Deref.
Clean up marker types and Unsafe initialization
auto merge of #13052 : sfackler/rust/clean-refcell, r=alexcrichton

These are superfluous now that we have fixed rvalue lifetimes and Deref.

I'd also like to kill off `get` and `set`, but that'll be a large change so I want to make sure that we actually want to do that first.