fix(library/unifier): flex_rigid case (?M ...) =?= (f ...), where f is not a constant nor a local

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add forall and exists theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add well-founded induction theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>