Added linked list to mol-data, refactoring

src/mol-data/generic.ts

src/mol-data/generic/_spec/linked-list.spec.ts

src/mol-data/generic/hash-set.ts

src/mol-data/generic/linked-list.ts

src/mol-data/generic/unique-array.ts

src/mol-data/index.ts

src/mol-data/util.ts

src/mol-data/util/array.ts

src/mol-model/structure/query/selection.ts

src/mol-model/structure/structure/structure.ts

src/mol-task/computation.ts

src/mol-task/execution/observable.ts

src/mol-task/index.ts

src/mol-task/task.ts

src/mol-task/util/chunked.ts

src/mol-task/util/multistep.ts

src/mol-task/util/now.ts