Reduce unnecessary Comparators in Range set

[isaacs]

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, and anything
that is >=2.3.4 is also >=2.3.0, so the <3.0.0-0 and >=2.3.0
Comparators are not necessary. But simplifying those out would be a bit
more work.

To do that, we could walk the set of Comparators checking to see if they
are a subset of any other Comparators in the list, and if so, removing
them. The subset check would not have to be a full Range.subset(); we
could just see if the gtlt points in the same direction, and if one
semver is greater than the other. It's an O(n^2) operation, but one on
typically very small n.

[coveralls]

Coverage remained the same at 100.0% when pulling 028a2f7 on isaacs/optimization-reduce-unnecessary-comparators-in-range-set into 226e6dc on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 499284b on isaacs/optimization-reduce-unnecessary-comparators-in-range-set into 226e6dc on master.