auto merge of #19157 : aturon/rust/cow-doc, r=alexcrichton

This commit makes `Cow` more usable by allowing it to be applied to
unsized types (as was intended) and providing some basic `ToOwned`
implementations on slice types. It also corrects the documentation for
`Cow` to no longer mention `DerefMut`, and adds an example.

Closes #19123

diff --cc src/libcollections/slice.rs
Simple merge

diff --cc src/libcollections/str.rs
Simple merge

diff --cc src/libcore/borrow.rs
Simple merge