feat(library/standard): add 'iff' and 'cast'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): allow 'assume'/'take'/'fun' as notation for apply tactic

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add iff_mp_right and iff_mp_left

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add exact_apply

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add exact_apply