suggestion of type: ustring

[ZY Zhou]

D's string is supposed to be utf8 encoded, however, the following code
compiles and runs with no error:

  string s = "\xff"; // s is invalid
  writeln(s);
  fileStream.writeLine(s);

In order to make sure only valid utf8 string is used in the system, validating
is needed everywhere, e.g.

  string cut3bytes(string s)
  in {validate(s);}
  out(result} {validate(result);}
  body {return s.length > 3 ? s[0..3] : s;}

I think it will be better if D has a ustring type to do all the validating
job. e.g.

  ustring s = "0xFF";  // compile error

  char[] c = [0xFF];
  ustring s = c.idup;  // throw UtfException

  ustring s1 = "\xc2\xa2";
  ustring s2 = s1[0..1];  // throw UtfException

So the above example can be simplified to:

  ustring cut3bytes(ustring s)
  {return s.length > 3 ? s[0..3] : s;}

--ZY Zhou