This PR fixes the behavior of `String.prev`, aligning the runtime
implementation with the reference implementation. In particular, the
following statements hold now:
- `(s.prev p).byteIdx` is at least `p.byteIdx - 4` and at most
`p.byteIdx - 1`
- `s.prev 0 = 0`
- `s.prev` is monotone
Closes#9439
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`.
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Co-authored-by: Kim Morrison <kim@tqft.net>