As Set is now a core collection class, it should have special inspect
output.  Ideally, inspect output should be suitable to eval, similar
to array and hash (assuming the elements are also suitable to eval):

  set = Set[1, 2, 3]
  eval(set.inspect) == set # should be true

The simplest way to do this is to use the Set[] syntax.

This deliberately does not use any subclass name in the output,
similar to array and hash. It is more important that users know they
are dealing with a set than which subclass:

  Class.new(Set)[]
  # this does: Set[]
  # not: #<Class:0x00000c21c78699e0>[]

This inspect change breaks the power_assert bundled gem tests, so
add power_assert to TEST_BUNDLED_GEMS_ALLOW_FAILURES in the workflows.

Implements [Feature #21389]