This provides a non-copying API, which is generally favourable. The
other accessors were removed, because the intent was to provide this
"most sensible" API via the "default" (i.e. shortest, most concisely
named) functions.
This introduces a str-like type for each of these, which will allow
returning a ref instead of the copied data from various methods in
{Full,Bare}Jid.
The use of a macro ensures that all types are declared consistently.
