dropped ES6 support for iterators

src/mol-base/collections/_spec/iterators.spec.ts

src/mol-base/collections/_spec/segmentation.spec.ts

src/mol-base/collections/integer/impl/segmentation.ts

src/mol-base/collections/iterator.ts

src/mol-data/_spec/atom-set.spec.ts

src/mol-data/atom-set/base.ts

src/perf-tests/iterators.ts

src/perf-tests/sets.ts