perf(core): have FilterMapOperator extend MapOperator
perf(core): have FilterMapOperator extend MapOperator

They seem to now share the same hidden classes and is now about
2.5x faster in the filter-map-fusion perf test

staltz/xstream,andersonleite/xstream,andersonleite/xstream,staltz/xstream,staltz/xstream,staltz/xstream,andersonleite/xstream