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.
Repeatedly calling str.replace is going to be slow. It'll be O(l * n) where l is the string length and n is the number of substitutions. It'd be slightly better to use `String.splitMapJoin`, however matching on a regex might not be sufficient, since we have to worry about escaping. C# uses `{{` to escape `{`, meaning that matches on `{\d*}` aren't all valid. I think the string needs to be processed rune-by-rune.