String Additions

String capitalization. Index-based insertion, replacement, and removal. Similar implementation of C# String.Format(..) to allow for terse index-based string interpolation without needing to use a literal with '$var' at the time of interpolation.
Possibly shorter (warning not tested for correctness):
```
  if (overwrite < 0) throw new ArgumentException(overwrite);
  if (overwrite == null) {
    overwrite = replacement.length;
  }
  var before = original.substring(0, index);
  var after = (index + overwrite > original.length) ?
      '' : original.substring(index + overwrite);
  return '${before}${replacement}${after}';
```