Implement `Borrow<str>` on JID types
This allows, via as_str(), to look up any JID type within a HashMap
or HashSet of any other JID type.
In an ideal world, we'd have proper ref types for the JIDs.
Unfortunately, that's not really possible: we would need an dynamically
sized type containing some data and then the string slice. Such a type
can be defined, but not constructed in stable safe Rust. A type like
FullJidRef<'_> would not suffice, because Borrow<T> requires to
return a reference, not an owned value.
Fixes #122 (closed).