Feature/dialect modules

@joshmarlow I've split the code base up to include a "dialects" module, where pybuiltins are pieces of the language that hail from python and norvig for the language constructs that hail from norvig's implementation. I'm planning on adding more configuration to the repl soon so that environs and command line arguments can affect runtime configuration of the language itself. This should allow me to avoid bike shedding in any language design decisions :)