[plt-scheme] MysterX bug squished

As reported on comp.lang.scheme, MysterX had a long-standing bug where
inherited methods and properties were not reported by com-methods,
com-get-properties, etc.

I've fixed the bug in the CVS code; the pre-release binaries contain the
fix.  As usual for COM, fixing the bug meant trying to trying to guess
what the COM documentation really means, and trying stuff until things
worked.

It's amazing that this bug hadn't surfaced before.  That suggests that
COM server implementors rarely use inheritance in their coclasses.

-- Paul