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 think this would be more flexible as `replace(String s, int index, String replacement, {int length})` where `length` defaults to `replacement.length`.

`replace('foo', 3, 'bar') => 'foobar'`
`replace('foo', 0, 'bar') => 'bar'`
`replace('foo', 0, 'bar', length: 0) => 'barfoo'`
`replace('foo', 1, 'bar', length: 2) => 'fbar'`

`insert` and `remove` would be sugar for calls to `replace`