[C++] Refactor Iterator to a type erased handle

[asfimport]

Since Iterator<T> is used as a base class for exported classes and is instantiated in both arrow.dll and arrow_dataset.dll we get multiple definition errors. The solution taken by MSVC's stl implementation in the similar case of std::function is type erasure. Since it's not a base class MSVC doesn't force public visibility of its members.

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Create reusable Iterator interface (supercedes)

PRs and other links:

• GitHub Pull Request #5391

_{Note: This issue was originally created as ARROW-6558. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 5391
#5391