Remove postinit

[ceridwen]

This is big but in many places boring. The brief summary of changes:

• Removed parent attribute from nodes.
• Removed postinit and combined its arguments into __init__.
• Made arguments for node __init__ calls mandatory, exept for lineno/col_offset.
• Made calls to node constructors use keywords for their arguments, to make the code easier to read.

There are still some TODOs in the code reflecting places where things either require more work that's already planned or I don't know how to solve them right now.