Old shape ran byteindex then rewound to count runes — two passes,
different algorithm from Hare. New `indexstring` mirrors
ref/hare/strings/index.ha:59-81: one outer iterator over the
haystack, an inner iterator re-seated from it for each candidate
match, both walking rune-by-rune. Returns the rune-index of the
first match, or void.
Rest-iterator copy is field-wise rather than `let rest_iter =
s_iter;` because the local-to-local copy of the 3-field iterator
struct diverges between stages today (#41 — 993_ww_ww and
995_self_rebuild byte-diverge when written the natural way).
WHY-comment cites #41 with the precise failing tests.
Tests pin the rune-vs-byte distinction at i=2 and i=4 with 3-byte
kana, plus self-match, empty-needle, empty-haystack, and a no-match
multibyte row from ref/hare/strings/index.ha:119.
Hojun-Cho
dd27ce3339