Fix R* lifetimes via `RCow` redesign

[marioortizmanero]

Closes #75

TODO:

• Update docs (look for RCow<'a and fix it)
• Fix tests & examples
• Is it possible to implement Deref for RCow<B> instead of for all of RCow<&'a B>, RCowStr<'a>, and RCowSlice<'a, B>? That way I can avoid duplicating other impls such as Display, and conversions in e.g., slice_like_impl_cmp_traits will continue to work. I think this is impossible, though. Probably not possible.
• It would be nice if #70 and #77 were merged to avoid possible conflicts
• I think I should make B: ?Sized too (?)
• Create some macros to avoid boilerplate? Though to be honest it's way simpler and less messy without them

[marioortizmanero]

Note that this PR also includes changes from #76 in order to also fix lifetimes for RVec and RHashMap. It also adds new ones, such as using NonNull in RVec; *mut T is invariant over T, but NonNull<T> is covariant over T.

[marioortizmanero]

I take it this can be closed as well?