Add a 'map<K,V>' type

[lukewagner]

This PR adds a map<K,V> to the set of Component Model and WIT value types as discussed in #125, specifically based on this design by @yordis. With the approach of treating map<K,V> as just a specialization of list<tuple<K,V>>, the addition is pretty small and most of the work will be in the bindings generators. I'm not in a particular rush to add this, but given the interest in #125, it seems useful to have a detailed proposal that can be prototyped and then potentially merged.