SI-7592 Replace s.t.n.u.TreeSet with s.c.m.TreeSet

This means
- migrating usages from the compiler-specific implementation to the
  one in the standard library
- removing the now unused compiler-specific implementation
Use forward slash in #2637's test on windows

Don't use toString to output file names in tests,
its output is platform dependent -- use `testIdent` instead.
Merge pull request #2663 from soc/SI-7592

SI-7592 Replace s.t.n.u.TreeSet with s.c.m.TreeSet
Merge pull request #2699 from adriaanm/fix-2637-windows

Use forward slash in #2637's test on windows