Implements some ABNF core elements for convenience:
- RuneIsALPHA(c rune)
- RuneIsBIT(c rune)
- RuneIsCHAR(c rune)
- RuneIsCR(c rune)
- StringIsCRLF(s string)
- RuneIsCTL(c rune)
- RuneIsDIGIT(c rune)
- RuneIsDQUOTE(c rune)
- RuneIsHEXDIG(c rune)
- RuneIsHTAB(c rune)
- RuneIsLF(c rune)
- StringIsLWSP(s string)
- RuneIsOCTET(c rune)
- RuneIsSP(c rune)
- RuneIsVCHAR(c rune)
- RuneIsWSP(c rune)
See RFC 5234 for more details.