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ARROW-8843: [C++] Compare bitmaps in words

Unaligned bitmap comparision are currently processed bit-by-bit.
Comparing word-by-word in uint64 improves performance significantly.
Bechmark(comparing two identical bitmaps with 64K bits) jumps from
86M/s to 5.7G/s on my test machine.

NOTE: This patch may hurt performance if two bitmaps differ at the very
begining bits, as it always loads and compares 64 bits if possible. Bit
by bit comparison will notice the difference and return false earlier.
This should not be a problem in practice.

Closes #7285 from cyb70289/bitmap-equals

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>