Use names like '$fOrdInt' for dfuns (and TF instances), rather than '$f21'

2 reasons for this:
  - compilation is more predictable.  Adding or removing an instance
    is less likely to force unnecessary recompilation due to
    renumbering other dfun names.
  - it makes it easier to read Core / C-- / asm

The names aren't completely deterministic.  To do that, we'd have to
include package and module names, which would make the symbol names
long and reduce readability.  So the compromise is that if there's a
clash, we disambiguate by adding an integer suffix.  This is fairly
unlikely in practice unless you're using overlapping instances.

Type family instances are handled in the same way, with the same
disambiguation strategy.