collate: make Collator.Compare and CompareString goroutine-safe

[carli2]

Collator stored two iterator slots as struct fields (_iter [2]iter).
Both Compare and CompareString wrote into these shared fields, making
concurrent calls on the same Collator instance unsafe.

Remove _iter [2]iter from Collator. Compare, CompareString,
getColElems, and getColElemsString now declare their iter as a
goroutine-local variable. Each goroutine gets its own stack-allocated
scratch buffer (wa [512]colltab.Elem); the CPU only loads the cache
lines the comparison actually touches, so early-exit comparisons pay
no extra cost. Collator is now immutable after construction.

Fixes golang/go#77927

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Patch Set 1:

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