Refactor Structure to use IntMap for units

src/mol-data/int/map.ts

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

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

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

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

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

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

src/perf-tests/structure.ts