Rename compiler, interpreter, serialize, and render

[fsaad]

compiler.py -> spml_to_python.py
interpreter.py -> ast_to_spn.py
serialize.py -> spn_to_json.py
render.render_spml -> spn_to_spml.py

[fsaad]

All of these are various "intermediate representations"; it is best not to frame the system in terms of compilation, interpretation, etc. just yet. One easy to explain workflow is:

where

1. "compiler" is the action of "spml_to_python" and "ast_to_spn"
2. "inference engine" is the action of "spn.py".