fix(src/builtin/tactic): add default rule set if none is provided

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/kernel): add another rewrite rule

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): add Jeremy's proof to test suite

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