data/bit-vector: change implementation to fxvectors
data/bit-vector: change implementation to fxvectors

There are minor benefits to using fxvectors instead of vectors:
the GC knows that an fxvector has no pointers inside, and the
bytecode compiler knows that `fxvector-ref' produces a fixnum.

BTW, a benefit of fxvectors over byte strings is the lack of an
indirection internally. (Unlike a byte string, the content of
an fxvector is "inlined" into the vector object.)

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