Add support for unfold <n>.

Semantics:

 - Plain "unfold" still means: first clause that matches
 - New "unfold n" means:
   - check that the definition has at least n clauses
     (error otherwise)
   - then attempt to unfold only the nth clause

Clauses are numbers from 1 to total number of clauses in that mutually
(co)inductive definition group.

Resolves #5.