[C++][Gandiva] Reduce use of Boost, remove Boost headers from header files

[asfimport]

Boost is currently a transitive dependency of many of Gandiva's public header files. I suggest the following:

• Do not include Boost transitively in any installed header file

• Reduce usages of Boost altogether

  On the latter point, most usages of Boost can be trimmed by having a hash_combine function inside the Arrow codebase. See results of grepping the codebase

  https://gist.github.com/wesm/190006d91628e6bf7c04deb596a52cff

  It seems that Boost cannot be easily eliminated altogether at the present moment because of a use of Boost.Multiprecision (int256_t). At some point someone may want to implement sufficient 256-bit integer functions so that we don't have to depend on Boost.Multiprecision

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-8623. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Only a usage of boost/multiprecision remains and outside of header files