feat(library/util): add get_intro_rule_names

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/abstract): add new abstract_local procedure

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic/inversion_tactic): adjust inversion tactic to HoTT lib
refactor(hott/init): 'prod' and 'sum' notation should be infix right like 'and' and 'or'
feat(hott/init/types): add 'sum' notation
feat(hott/init/nat): add basic facts about natural numbers
test(tests/lean/hott): add test for 'cases' tactic