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.
Add tests for edge cases such as:
```
expect(insert('', 0, 'abc'), 'abc');
expect(insert('abc', 1, ''), 'abc');
```
and for failure cases such as:
```expect(() => insert('ab', 5, 'c'), throws);```