[C++] Hash table specializations for small integers

[asfimport]

There is no need to use a dynamically-sized hash table with uint8, int8, since a fixed-size lookup table can be used and avoid hashing altogether

Reporter: Wes McKinney / @wesm
Assignee: Panchen Xue / @xuepanchen

PRs and other links:

• GitHub Pull Request #1551

_{Note: This issue was originally created as ARROW-1942. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1551
#1551