Standalone ASDL parser for upstream CPython 3.x.

The parser is in a single file - asdl.py; it contains a hand-written lexer and a recursive-descent parser.

Note: Python.asdl (the ASDL definition file for Python) and asdl_c.py (emitter for Python-ast.[hc]) are copied over from the CPython repository (default branch); I applied some very small cleanups to asdl_c.py, mainly because asdl.py produces cleaner ASTs than the old Spark-based parser. When run, it produces exactly the same Python-ast.[hc] as in upstream CPython.

The officially required version is Python 3.3, but should run with any 3.x

Same as CPython: Python Software Foundation License (LICENSE file included here).