Var analysis data naming

[Reid D McKenzie]

https://github.com/arrdem/oxcart/blob/develop/src/oxcart/passes/tree_shake.clj#L85-L112 is inconsistent with https://github.com/arrdem/oxcart/blob/develop/src/oxcart/passes/tree_shake.clj#L156-L176, and the latter is arguably clearer that the analysis data is specific to vars and that the analysis involved is global. A fixup of the former is in order at some point.