This PR enforces rules around arithmetic of `String.Pos.Raw`.
Specifically, it adopts the following conventions:
- Byte indices ("ordinals") in strings should be represented using
`String.Pos.Raw`
- Amounts of bytes ("cardinals") in strings should be represented using
`Nat`.
For example, `String.Slice.utf8ByteSize` now returns `Nat` instead of
`String.Pos.Raw`, and there is a new function `String.Slice.rawEndPos`.
Finally, the `HAdd` and `HSub` instances for `String.Pos.Raw` are
reorganized. This is a **breaking change**.
The `HAdd/HSub String.Pos.Raw String.Pos.Raw String.Pos.Raw` instances
have been removed. For the use case of tracking positions relative to
some other position, we instead provide `offsetBy` and `unoffsetBy`
functions. For the use case of advancing/unadvancing a position by an
arbitrary number of bytes, we instead provide `increaseBy` and
`decreaseBy` functions. For
offsetting/unoffsetting/advancing/unadvancing a position `p` by the size
of a string `s` (resp. character `c`), use `s + p`/`p - s`/`p + s`/`p -
s` (resp. `c + p`/`p - c`/`p + c`/`p - c`).
dca8d6d188