perf(dropRepeats): set equality function only once
perf(dropRepeats): set equality function only once

Little optimization to extra/dropRepeats to set equality function only inside constructor, not
checking it on every call to isEq, plus some bytes saved in code and deleting an unused public
variable (this.fn) from class.

PR #213
staltz/xstream,staltz/xstream,staltz/xstream,staltz/xstream