[C++] Fix compiler warnings on 32-bit platforms

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

_{Note: This issue was originally created as ARROW-588. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
I started looking at this. Fixing all the -Wconversion warnings on 32-bit probably has diminishing returns, because anytime we use an int64_t as an index (e.g. with std::vector), it complains about a cast to std::vector<T>::size_type.

[asfimport]

Wes McKinney / @wesm:
PR: #354

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 354
#354