Added the following (all unstable):
* Defaulted type pararameter `A: Allocator`.
* `UniqueRc::new_in()`.
* `T: ?Sized` where possible.
* `impl CoerceUnsized for UniqueRc`.
* Drive-by doc polish: links and periods at the end of sentences.
These changes are motivated by supporting the implementation of unsized
`Rc::make_mut()` (PR #116113), but are also intended to be obvious
generalizations of `UniqueRc` to support the things `Rc` does.
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