Turn Oil's expression grammar into an AST

[andychu]

Two parts to this:

• Copy Python's AST from Python.asdl into our frontend/syntax.asdl.
• Write the "transformer" from pgen2's parse tree to the Python AST. (This code replaces semantic actions in most parsing systems.)

Not part of this task:

• Write a dirt simple tree evaluator for the AST. (Copying Python's AST gives me some confidence that it's "right" without actually writing the evaluator.)

[andychu]

Example: 49b0649

[andychu]

Done awhile ago with oil_lang/expr_to_ast.py