unsafe fn prefilter_kind_fallback(
    strat: &Prefilter,
    haystack: &[u8],
) -> Option<usize> {
    strat.kind.fallback.find_prefilter(haystack)
}
Reads from the `fallback` field of `PrefilterKind` to execute the fallback
prefilter. Works on all platforms.

# Safety

Callers must ensure that the `strat.kind.fallback` union field is set.
src/memmem/searcher.rs