fix(frontends/lean): fail if theorem type has metavariables after type elaboration (and before proof elaboration)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): assertion violation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean): adjust test to reflect recent changes

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