Use map instead of dict in riak_dt_map

[jessestimpson]

Draft for discussion, not intended to be merged.

It was fairly mechanical to replace dict with map, and keep unit tests passing. No other testing was done.

Known issues:

• Dialyzer throws an error because the code is using the knowledge that a dict is a tuple, which is a violation of the opaque type.
• I elected to use version 3 to represent the map-map implementation. I don't know what the implications of this are.
• Need new tests for converting from v2 to v3