b51c02f93b
Antlr root nodes need to implement RootNode. Also, I didn't like the fact that the swift module didn't use the conventional package structure (lang.swift.ast) but was rolling its own convention (lang.swift.antlr4). I moved base classes for the antlr implementations into nspmd.lang.ast.impl.antlr4. The fact that a module is implemented with antlr is an implementation detail, and it doesn't deserve its own toplevel package in nspmd.lang.