A custom AST hierarchy for our own use

[lihaoyi]

The default python AST objects are clunky and annoying:

• They don't have nice __str__s or __repr__s
• They don't have a constructor that will work with pattern matching
• They don't have structural equality
• Behave differently in CPython, PyPy and Jython and are probably the main reason why Jython doesn't work right now.

In general they are a pain to use. It would be great if we could have a different ast representation, either using astng or made ourselves using case classes.

[ceridwen]

I don't understand how the absence of infer_call_result has any relevance to MacroPy, it looks like it's there for checking function return/call sign signatures for Pylint.