ARROW-8793: [C++] Do not inline BitUtil::SetBitsTo

[wesm]

Local benchmarks suggest that there is 3-4 ns of overhead for manipulating small bitmaps when it is not inline.

inline (argument is byte size of bitmap):

--------------------------------------------------------
Benchmark Time CPU Iterations
--------------------------------------------------------
SetBitsTo/2 3 ns 3 ns 271439203 733.294MB/s
SetBitsTo/16 2 ns 2 ns 308813758 6.49485GB/s
SetBitsTo/1024 9 ns 9 ns 79821710 109.078GB/s
SetBitsTo/131072 2029 ns 2029 ns 325563 60.1566GB/s

non-inline:

--------------------------------------------------------
Benchmark Time CPU Iterations
--------------------------------------------------------
SetBitsTo/2 6 ns 6 ns 129335891 334.62MB/s
SetBitsTo/16 6 ns 6 ns 122741527 2.53134GB/s
SetBitsTo/1024 11 ns 11 ns 64547137 87.1395GB/s
SetBitsTo/131072 2010 ns 2010 ns 332558 60.7215GB/s

If it can be demonstrated that inlining can meaningfully improve the macroperformance of some function, then we could lift the implementation into a SetBitsToInline, but until then I think it makes sense to keep it in bit_util.cc

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8793

[wesm]

+1