Update PatternConverter

[Azoy]

This updates the pattern converter to generate DSLs that are updated with the current proposed (and implemented) API. There are some character classes that are "unprintable" which kind of complicate the printing process.

// For example: '[\d[:upper:]abc](a|b)'

Regex {
  #/\d[:upper:]abc]/#
  Capture {
    ChoiceOf {
      "a"
      "b"
    }
  }
}

That being said, character classes that we support print pretty well!

// For example: [a-z--[^a-c]]

Regex {
  ("a"..."z")
    .subtraction(("a"..."c").inverted)
}

[milseman]

Quick look over and LGTM

[DaveEwing]

Why not print them as a comment (since that's what they are)?

[Azoy]

@swift-ci please test