Liveness analysis operates on individual functions.
Set parent links.
Allow to get function from module by name: mod["func"].
Make N constant, as in the Appel book.
Make PInstruction.succ() work even if block doesn't end in ctrl xfer.

In this case, it just returns first instruction in the next block.

This relaxation is helpful dealing with examples from books, etc.
Add index()/extended __getitem__() for PFunction.