Switch to storing AST nodes on the stack for more accurate method signature check and easy access to parent nodes

[stefmolin]

AST nodes have knowledge of their children, but not their parents which is why we use a stack in the DocstringVisitor. Currently it is used to track a node's unambiguous name (e.g., my_module.MyClass.__init__). This PR changes that to store the AST nodes themselves, which allows us to use a node's ancestry for more robust logic. For example, with this change, we can distinguish between a function and a method, and more precisely determine whether to ignore self and cls parameters in a signature.