Adds guarded ref and mut access for UnsafeCell

[kylefleming]

This PR adds methods guard_ref_access and guard_mut_access to UnsafeCell as an alternative to with and with_mut for tracking read and write access to the underlying cell data.

The 2 methods return tokens (UnsafeCellRefToken and UnsafeCellMutToken, respectively) that implement Drop so that the beginning of the access can be marked by the invocation of the guard_*_access methods and the end of the access is marked by the release of the token object.

I appologize in advance for the lack of proper documentation and tests to accompany the feature. However, I did add doc comments.

This PR also allows the T parameter of UnsafeCell<T> to be unsized (T: ?Sized) to mirror std::cell::UnsafeCell, although I can break that out into a separate PR if needed.