Strip off leading dots and slash so the generated files are the same regardless
of whether you configure in the checkout directory or build.
If anyone configures in a different directory, we might want a cleaner
approach using os.path.*(). Hopefully this is good enough.
This change implements a new bytecode compiler, based on a
transformation of the parse tree to an abstract syntax defined in
Parser/Python.asdl.
The compiler implementation is not complete, but it is in stable
enough shape to run the entire test suite excepting two disabled
tests.