Added conditional map support for mochijson2:encode/2

[marjaimate]

• Platform define map_available added to allow conditional support for maps
• Expand the encode/2 functionality to parse maps
• Added json_encode_map/2 to handle map parsing, similar to arrays and proplists
• Unit tests for map encoding

[etrepum]

This is now available in the tagged v2.16.0 release