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GH-36059: [C++][Compute] Reserve space for hashtable for scalar lookup functions (#36067)



### Rationale for this change

Performance improvement for scalar lookup functions.

### What changes are included in this PR?

1. Reserve space for hashtable for scalar lookup functions before the insertion process.
2. A benchmark that demonstrates the improvement. Data is pasted in the comments.

### Are these changes tested?

By existing unit tests.

### Are there any user-facing changes?

No.
* Closes: #36059

Lead-authored-by: Jin Shang <shangjin1997@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>