Make TmOracle reduce nullary constructor equalities

Previously, `simplifyEqExpr` would just give up on constructor
applications when no argument to either constructor was simplified.

That's unfortunate as all arguments might be equal anyway! A special
case of this is nullary constructors. Currently, the TmOracle would fail
to solve a term equality `False ~ (True = True)` because it can't make
any progress on any of `True`s arguments.

Instead we now try to properly simplify the term equality even when no
simplification of constructor arguments was achieved.