I realized that if you ever wanted to the way taint-steps works again,
you would have to go to all the 117 places it has been implemented, and
change EVERY ONE OF THEM :( so trying to solve that problem here.
Not super happy with the name, but that was just the best I could come up with :D
I know that the TODO about not having the tools to handling
`meth = obj.meth; meth()` is outdated now that we `DataFlow::MethodCallNode`,
but I'm planning to deal with that later on ;)
Splits `ModuleVariableNode` away from `LocalSourceNode`, instead
creating a class `TypeTrackingNode` that encapsulates both of these.
This means we no longer have module variable nodes as part of
`LocalSourceNode` (which is good, since they have no "local" aspect to
them), and hence we can have `LocalSourceNode` inherit directly from
`ExprNode` (which makes the API a bit nicer).
Unfortunately these are breaking changes, so we can't actually fulfil
the above two desiderata until the `track` and `backtrack` methods on
`LocalSourceNode` have been fully deprecated. For this reason, we
preserve the present implementation of `LocalSourceNode`, and instead
lay the foundation for switching over in the future, by deprecating
`track` and `backtrack` on `LocalSourceNode`.
Rasmus Wriedt Larsen
Taus