BigString sometimes miscounts distances in its character view due to alignment issues.

let big = BigString("a" + String(repeating: "\u{301}", count: 300))

print(big.count) // 1 ✔️
print(big.distance(from: big.startIndex, to: big.endIndex)) // 2 ❌

// If `big` had two characters, then the index after the start should not be at the end
let i = big.index(after: big.startIndex)
print(i == big.endIndex) // true ✔️

print(Array(big[...])) // Trap: invalid collection 💥

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.

rdar://153412693

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.