Lifetime specifications on type parameters
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Should it be possible to...

* Specify lifetimes involving type parameters?
* Specify that two type parameters have the same type,
   but different lifetimes or different qualifiers (var/threaded/etc)?

My guess is that the answer is NO to both questions,
or at least that it would complicate the compiler implementation
to support either (or both) of these two features.

Examples:

    # Would it be meaningful to specify a lifetime constraint involving T?
    type Thing<T>

    # Would it be meaningful to specify lifetimes on a and/or b?
    # Would it be meaningful to specify qualfiers on a and/or b?
    func Thing<T>.do_stuff(slot T a, slot T b)