Fix == rewriting on embedded terms

In a660d8f2 we added a rewriting stage that converts == to unification.
This simplifies evaluation and allows the rule index to be used,
however, the rewriting was incorrectly applied to == expressions that
are intended to yield a true/false value (and not undefined).

Fixes #995

Signed-off-by: Torin Sandall <torinsandall@gmail.com>