Use discrete interval trees for character classes

[ceedubs]

Resolves #69 and #73.

[codecov-io]

Codecov Report

Merging #75 into master will decrease coverage by 3.28%.
The diff coverage is 91.52%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   98.91%   95.62%   -3.29%     
==========================================
  Files          17       19       +2     
  Lines         368      366       -2     
  Branches        7       10       +3     
==========================================
- Hits          364      350      -14     
- Misses          4       16      +12

Impacted Files	Coverage Δ
parser/src/main/scala/Parser.scala	100% <ø> (ø)	⬆️
regex-gen/src/main/scala/DietGen.scala	100% <100%> (ø)
regex/src/main/scala/Regex.scala	100% <100%> (ø)	⬆️
regex-gen/src/main/scala/CharRegexGen.scala	100% <100%> (ø)	⬆️
regex/src/main/scala/RegexPrettyPrinter.scala	100% <100%> (ø)	⬆️
regex-gen/src/main/scala/RegexGen.scala	100% <100%> (ø)	⬆️
regex-gen/src/main/scala/RegexMatchGen.scala	100% <100%> (ø)
regex/src/main/scala/CharacterClasses.scala	100% <100%> (ø)	⬆️
regex-gen/src/main/scala/RegexShrink.scala	57.14% <18.18%> (-36.61%)	⬇️
regex/src/main/scala/Match.scala	82.35% <81.25%> (-5.15%)	⬇️
... and 3 more

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