sturcuture factoring, queries

src/mol-data/structure/query/generators.ts

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

src/mol-data/structure/structure/atom.ts

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

src/mol-data/structure/structure/atom/set/builder.ts

src/mol-data/structure/structure/atom/set/impl.ts

src/mol-data/structure/structure/structure.ts

src/perf-tests/sets.ts

src/perf-tests/structure.ts