Diff: store depth on node prior to minimizing

[zachallaun]

Node depth is currently calculated dynamically. It needs to be cached in the node meta prior to minimizing because minimizing can hoist deeper nodes up.

This is technically a bug.

Also a good opportunity to be more precise about depth. It should be syntactic depth, not AST depth, I think. For instance, 2 element tuples representing kw pairs should be ignored for depth, I think.