Eager evaluator should memoize intermediate results

Sometimes calculated expressions may compute the same column or subexpression multiple times. For example, in `{"sum": f.A + f.B, "dif": f.A - f.B}` each of the columns `A` and `B` is extracted from the source Frame and materialized twice, even though we could have remembered the result on the first run. A similar mechanism is already used for llvm evaluator, so this should be relatively easy to implement.