VariableCollector takes an ABT and builds a vector of all the Variable references (and definitions). Sometimes the consumer doesn't want a vector: they may want a set, or a per-variable count.

VariableEnvironment::walkVariables() calls a callback for each Variable, which lets the consumer decide what collection to build (if any). But currently it uses VariableCollector, which builds a vector. Instead we should have a different transport that takes a callback.