feat(emacs): minor improvements to emacs mode

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/kernel): add eqf_intro and eqf_elim theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/kernel): 'implication' simplification theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/congr): add congruence theorems for contextual simplification

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