feat(library/tactic): add disj_hyp_tactic

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): add absurd_tactic

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic): remove duplicate code, add add_proofs

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