Allow indirect/partially owned mapping

[ilyvion]

I don't know if the title is all that self-explanatory, but it's the best I could think of, so let me instead try to explain what I mean here:

Today, RwLockReadGuard takes a closure with the signature FnOnce(&T) -> &U, which means that the only thing you can do is return a borrowed value. This means you can't do something like

struct MyCustomType<'a> {
    value: &'a Value,
}
// used with `map` like:
RwLockReadGuard::map(guard, |value| MyCustomType {
    value,
})

since, although this is still a sensible mapping, it does not fit the shape of a &U and will be rejected.

[Amanieu]

This was previously raised in #289/#290. Basically, the issue is that Rust's lifetime system can't properly support this at the moment.