kelindar / bitmap / 35854983487

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Fix Or/Xor dropping bits when combining bitmaps of different lengths (#51)

* fix: union/xor of different-sized bitmaps drops bits in variadic path

Or and Xor with extra bitmaps used _or_many/_xor_many, which apply a single
word count to every input. The count came from pointersOf, whose length is
the maximum over the extra slice only and ignores other, so a longer other
lost its tail words. More generally a uniform count cannot combine operands
of different lengths: a longer source is truncated and a shorter one is read
past its end.

Fold each source in with the single-operand SIMD primitive over its own
length, matching the scalar reference in simd_generic.go. Fixes the amd64
accelerated and AVX-512 branches and the arm64 branch.

* Keep the _many primitives for equal-sized operands, fold only ragged ones

* test: cover longer ragged bitmap extras

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Co-authored-by: Roman <roman.atachiants@gmail.com>

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