refactor(library/tactic): move 'exact' tactic to separate module
feat(library/tactic): add new approach for embedding non-elaborated expressions into tactics
refactor(library/tactic/rename_tactic): use new 'tactic.expr' to implement rename tactic
refactor(library/tactic/rename_tactic): use new 'tactic.expr' to implement 'intro/intros' tactic

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