Add simple visitor for expressions

[westonpace]

We should be able to considerably simplify the process of visiting a substrait expression. We can categorize most nodes into one of a few kinds (function call, literal, field reference), can handle the anchor lookups, etc.

Might need a few variants for pre/post visitors and visitors that return vs do not return.

This kind of thing is usually easiest when done in the context of some kind of goal. So it might be better to write something that probably needs a visitor like expression transformation first.