Add '--parse-only' option.

When this is set, don't construct a DAG, just parse the AST.
This should be virtual.

We don't have any subclasses yet, but we will very soon...
Add join(vector<string>, string delimiter).

This is like Python's ', '.join(some_list).
Let an sat::Visitor know about Scope visiting.
Provide count of type parameters.

This is pretty essential when using operator[]!
Add support for concatenating string lists.