a file's name is folded once, not once per name test

Folding is the expensive half of a name test on a name with diacritics,
and every name test of every rule folded the same name again: on Polish
names it was most of the matching work. The per-file facts memoise it,
which is where one file's work belongs - the object is per file and per
goroutine, so no lock.

  4000 Polish names, twelve rules with name tests: 0.33s -> 0.13s

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