String
Add docstrings, usage examples, and doctests for `String.get'`, `String.next'`, `String.posOf`, `String.revPosOf`.
Add docstrings, usage examples, and doc tests for `String.prev`, `.front`, `.back`, `.atEnd`. Improve docstring examples for `String.next` based on discussion examples for `String.prev`. --------- Co-authored-by: Kim Morrison <kim@tqft.net>
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>