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.
I did a very nasty-looking loop to go rune by rune, I was hoping to get some input on the logic. It really hurts to look at.. works.. but I don't know if the performance is any better with this approach. https://gist.github.com/Term1nal/780405975e0a0622ee1a