Sequence as more 1st class citizen

src/apps/structure-info/model.ts

src/mol-data/db/column.ts

src/mol-model/sequence.ts

src/mol-model/sequence/constants.ts

src/mol-model/sequence/sequence.ts

src/mol-model/structure/model/formats/mmcif/sequence.ts

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

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