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Reduce unnecessary Comparators in Range set

There are a few cases that lead to Range's Comparator sets being longer
than strictly necessary.  This reduces performance of methods that
iterate over ranges repeatedly (for example, `intersects` and `subset`),
and leads to some confusing toString output like turning `x || * || X`
into `||||` instead of `*`.

- If any simple range in the set contains the null set <0.0.0-0, then
  the entire simple range is the null set. `2.x <0.0.0-0` is the same as
  just `<0.0.0-0`.  (This is used for `>*` and `<*`, which cannot match
  anything.)
- Ensure that a given Comparator will only occur once within each simple
  range set.  `2.3.x ^2.3` doesn't need to include `>=2.3.0` more than
  once.
- If a simple range set contains more than one comparator, remove any `*`
  comparators.  `* >=2.3.4` is the same as just `>=2.3.4`.  This was
  already being done in the cast to a string, but some `ANY` Comparators
  would be left behind in the set used for matching.
- If a Range set contains the simple range `*`, then drop any other
  simple ranges in the set. `* || 2.x` is the same as `*`.

There's still some unnecessary comparators in there.  For example, the
range `2.3 ^2.3.4` parses to `>=2.3.0 <2.4.0-0 >=2.3.4 <3.0.0-0`.  Of
course, anything that is `<2.4.0-0` is also `<3.0.0-0`, so that last
Comparator is not necessary.  But simplifying those out would be a bit
more work.

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