Prototype introiff option for find_theorems.
This feature was suggested by Jeremy Avigad / Tobias Nipkow.
It adds an introiff keyword for find_theorems that returns, in
addition to the usual results for intro, any theorems of the
form ([| ... |] ==> (P = Q)) where either P or Q matches the
conclusions of the current goal. Such theorems can be made
introduction rules with [THEN iffDx].
The current patch is for evaluation only. It assumes an
(op = : 'a -> 'a -> bool) operator, which is specific to HOL.
It is not clear how this should be generalised.