erl_types: Add a map type representation

The type of a map is represented as a three-tuple {Pairs, DefaultKey,
DefaultValue}. DefaultKey and DefaultValue are types. Pairs is a list of
three-tuples {Key, mandatory | optional, Value}, where Key and Value are
types. All types Key must be singleton, or "known at compile time," as
the EEP put it. Examples:

    #{integer()=>list()}       {[], integer(), list()}
    #{a=>char(), b=>atom()}    {[{a, optional, char()},
                                 {b, optional, atom()}],
                                none(), none()}
    map()                      {[], any(), any()}

A more formal description of the representation and its invariants can
be found in erl_types.erl

Special thanks to Daniel S. McCain (@dsmccain) that co-authored a very
early version of this with me back in April 2014, although only the
singleton type logic remains from that version.

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