DSL: add list and map types

[cesarblum]

Addresses #337

Can now do

data MyData {
  list<string> aList;
  map<string, int32> aMap;
}

And

actor MyActor {
  map<string, int32> aMethod(list<int64> arg);
}

Works with user-defined types as well, and can be nested (e.g. list<map<string, list<int32>>>).

Very rudimentary support with essentially no validation for now. I'll add validation in a subsequent PR.

[victorhurdugaci]

LGTM. A few comments but nothing critical