Map empty start/end index to 0/length for slice expressions

This allows doing the following:

public static int main (string[] args) {
    string a = "appelflap";
    stdout.printf ("%s\n", a[-4:]); // flap
    stdout.printf ("%s\n", a[:5]);  // appel
    stdout.printf ("%s\n", a[:]);   // appelflap
    return 0;
}

Fixes #238 (closed)