BigString sometimes miscounted distances in its character view due to alignment issues. Resolve this by overhauling distance calculations in the character metric:

- Introduce a `prefixSize` function in `_StringMetric`, distinct from `distance`.
- In the metric-agnostic index distance algorithm, use `prefixCount` to avoid double-counting characters that cross (or end on) chunk boundaries.
- Simplify `_StringMetric.distance` by requiring that `start <= end`.
- Crucially, change character-index operations to start by rounding each input index down to the nearest character break.

[BigString] Fix character indexing operations

This implements a roughly 40-45% improvement for distance measurements over `BigString`’s character and Unicode Scalar views.

Interestingly, we don’t see an improvement for UTF-16 views — I’m guessing this is due to String’s breadcrumbing. (And it will go away with the upcoming UTF8Span-ification of this type.)

The benchmarks I added only show an improvement for multi-chunk big string instances, but this is merely an artifact of the specific algorithm that is being measured — for single-chunk strings, the existing measurement method is already optimal, so the new heuristic never triggers for this particular payload.

[BigString] Harvest some low-hanging performance fruit