Propagate :locals down the parse tree

[mkremins]

Rather than determining locals by walking up the tree from a particular loc like we do right now, it might make more sense to populate :locals on each parse tree node, starting at the top of the tree with an empty set and introducing new locals as binding nodes are encountered.

Like the annotation of :head? nodes, this task would probably be performed during the render phase.

[mkremins]

This is probably a prerequisite for #21.