[C++] Implement Map logical type

[asfimport]

A map is implemented as a list of structs with fields key and value. We should separately discuss whether this merits an addition to the Arrow metadata

Reporter: Wes McKinney / @wesm
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Implement MapType, MapArray, MapBuilder (duplicates)
• [Integration][Java] Integration tests for Map type (is depended upon by)

PRs and other links:

• GitHub Pull Request #4352

_{Note: This issue was originally created as ARROW-1207. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Are there specific conventions or strong opinions about how this map type should be represented in a string? I'm leaning toward: map<utf8, int32> and map<int32, float64, keys_sorted> for unsorted/sorted keys respectively. Alternatively, I could follow dictionary type in verbosity by using map<keys=utf8, values=int32, keys_sorted=false> and map<keys=int32, values=float64, keys_sorted=true>.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 4352
#4352