refactor(builtin/heq): remove axiom hpiext since we don't use it anymore

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): add simplify_tactic based on the simplifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): add simplify_tactic based on the simplifier