Rewrite pattern parser

[mmastrac]

• Rewrote the pattern parsing to avoid using regular expressions, making all
  regular expression engine features optional (though at least one is still
  required).
• Ensure correctness in capture names, even when using duplicate names
  generated from pattern names. These were previously unspecified:
  • (breaking) Duplicate pattern names result in matches with incrementing names in the format:
    NAME, NAME[1], NAME[2], etc.
  • (breaking) Duplicate pattern names from aliases are now guaranteed to be
    "last one wins".
• (breaking) Matches::len() was removed as it was previously reporting the
  pattern name count. Use Matches::iter().count() instead.
• (breaking) Matches::is_empty() was removed. Use Matches::iter().count() == 0 instead.