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.
