Add extension point to resolve_variable_ast.

I'm back ;)

It's been a while, but I've gotten back trying to get our new extension up-to-date..

I hope this doesn't break anything. I ran the tests without errors, so it should be fine; if not, there's room for more tests :p


--From commit message--

For this, we need to pass the TreeWalker to resolve_variable_ast,
which affects the filter and cycle functions as well..

Allow the extension module to provide it's own find/fetch value runtime function.