LUCY-295 BitVector size_t ticks

[rectang]

Change BitVector to use size_t instead of uint32_t to address bits in the array.

[nwellnhof]

Next_Hit and To_Array still work with int32_t indices. For now, we should at least add a check like

if (ivars->cap > INT32_MAX) { THROW(...) }

In the long run, I'd suggest to change the return value of Next_Hit, remove To_Array, and switch its users over to Next_Hit.

[nwellnhof]

The overflow check should be:

if (bit_size > SIZE_MAX - 7) {
    return SIZE_MAX / 8 + 1;
}

Or maybe an alternative implementation:

static CFISH_INLINE size_t
SI_octet_size(size_t bit_size) {
    if (bit_size == 0) { return 0; }
    return (bit_size - 1) / 8 + 1;
}