Bulk-scan single-line string bodies

[tfoutrein]

Stacked on #489 and #490 — the first two commits here are those PRs (index-based Source + bulk run scanning). Best reviewed/merged after them; GitHub will collapse this to just the string-scan commit once they land. Happy to rebase.

What

Parsing a single-line string appended its body one character at a time (value += current; inc()). For long string values (the bulk of most config / lockfile content) this dominates.

This scans the run of ordinary characters up to the next delimiter, backslash or control character in a single pass (Source.advance_until) and appends the whole slice at once; the stop character is then handled by the existing branch on the next iteration. Multiline strings keep the per-character loop (CRLF handling).

The stop-set is exactly the control characters the per-character loop rejects, so InvalidControlChar / escape / delimiter handling is unchanged, and a mid-string EOF raises UnexpectedEofError just as the per-char inc(exception=...) did.

Benchmarks

Parsing speedup across document shapes (median, interleaved A/B vs master, includes #489+#490):

document	speedup
large flat, single-line string values (~90 KB)	4.9×
pyproject.toml	2.1×
poetry.lock-like (~64 KB)	2.1×
typical mixed (~4 KB)	1.5×

No regression on any shape (multiline-heavy and nested docs are unchanged).

Tests

Full suite passes (972 tests, incl. the toml-test conformance submodule). On top of that, a 4135-input adversarial differential (random escapes valid+invalid, every control byte 0x00–0x1F+DEL in basic & literal, unicode/astral/combining, the other-quote char inside strings, truncated/malformed inputs for error parity) is byte-identical in output and exception type to the per-character loop. No public API or behaviour change.

[frostming]

@tfoutrein please resolve the conflicts

[tfoutrein]

Rebased onto master now that #490 has merged — this is a single commit on top of current master, no conflicts. CI is green. Ready for squash-merge whenever you'd like. Thanks!