Move RuntimeLong.fromString directly into j.l.Long.parseLong.
Remove all methods from the RuntimeLong companion object.

Except fromDouble(), which forwards to RuntimeLongImpl.

These methods were used by the code generated by the compiler,
but they are not used by the new code generated by desugar,
because they were inlined "by hand" in this case.
Merge RuntimeLongImpl back into RuntimeLong.
Use the cached RuntimeLong.Zero for literal 0's.