Add `unfold named_clause with ...` form

[chaudhuri]

Todd Wilson (@lambdacalculator) suggests in the Abella mailing list that we should add a unfold named_clause with ... form to handle cases such as:

%:tapp:
of (tapp P E) (T P) :- typ P, of E (all T).

Here an invocation such as unfold tapp with T = ... might allow the clause to be potentially unfolded because it is no longer non-pattern after the substitution.