objtype: Don't treat inheritance from "object" as from native type.

"object" type in MicroPython currently doesn't implement any methods, and
hopefully, we'll try to stay like that for as long as possible. Even if we
have to add something eventually, look up from there might be handled in
adhoc manner, as last resort (that's not compliant with Python3 MRO, but
we're already non-compliant). Hence: 1) no need to spend type trying to
lookup anything in object; 2) no need to allocate subobject when explicitly
inheriting from object; 3) and having multiple bases inheriting from object
is not a case of incompatible multiple inheritance.
objset: Give up and implement frozenset.

Tired of patching CPython stdlib for it.
py: Disable frozenset by default, enable on unix.

Takes 416 text bytes on x86.
modsys, unix: Add sys.exit(), should be implemented by a port.
stmhal: Implement draft version of sys.exit().
run-tests: Add support for skipping tests.

MicrpPython test should print single "SKIP" line for test to be skipped.
objset: Add frozenset tests, skippable if frozenset not available.