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.
This would be fine as a normal function (and I think we can name it simply format, since it's in the strings libaray. Users can prefix if needed): `format(String template, Iterable items)`