DataType::Map tracking issue

[nevi-me]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We are adding support for the map data type, which is a list of key-values ala hashmap.
This issue tracks outstanding functionality without opening multiple issues for each at once.

The outstanding work after #395 and #491 are:

• Integration testing (https://issues.apache.org/jira/browse/ARROW-13300)
• Non-canonical map support (where we don't check that the schema names match exactly)
• Nested maps [arrow, parquet (write)]
• improving map builder support to behave like a hashmap
• JSON writer

[tustvold]

I believe all the listed items are now implemented so closing, feel free to reopen if I am mistaken