Support for Maps (WIP)

[bvandenbos]

This is early support for a map type. At this stage, I'm mostly looking for feedback and thoughts on whether or not you're interested in eventually supporting maps.

message PhoneBook {
  map<string, Person> phoneLookup = 0;
}

Some caveats:

1. It doesn't support nested maps (although neither do arrays)
2. I've only added support for JS/TS.
3. It only supports strings for keys for now. Values can be anything.
4. I'm pretty unhappy with the code.

[kevzettler]

Just ran into a need for this myself curious to @evanw's opinion