What kind of string type to support?

1. Only one type, which is UTF-8.
      - operations like toLower() and slice() will be expensive
      - when the string is actually arbitrary bytes there is no way to give
         an error when character based operations are used.

2. Support an utf-8 string type and byte string
    On both ASCII functions can be used, e.g. asciiToLower()
      + separate meaning of byte sequence from character sequence
       -  when only ASCII text is possible, operations like toLower() and slice()
          are still expensive

3. Support an utf-8 string type, an ascii string type and byte string
       + separate meaning for the three types
       + possible to use efficient operations
        - three types make it more difficult for the programmer

Choice 2: Strings are used very often, it's worth making them efficient.