Revert "AST: Upgrade FunctionDef to ListNode."

This reverts commit 8faaa87fc2a2aff85598a7d496f598fe823ecce5.

Conflicts:
	compiler/ast.py
Symbol: Avoid catch impossibl KeyError exception.
Symbol test: Refrain from instantiating RedefIdentifierError.
Symbol test: Thoroughly examine the raised RedefIdentifierError.