Remove xnum conversions

[juntuu]

This continues from #200

This removes the conversions from and to XNUM, and any cases triggering those in the tests #111

XNUM will still be present as building block for some of the other types

Identifying the conversions to and from XNUM was quite simple once those were removed from the code (https://github.com/codereport/jsource/issues/105#issuecomment-773707530). I just put an abort() where ever an XNUM conversion was, to disambiguate the test failure mode.