refactor(library/data/sigma): cleanup module
feat(library/data/category.lean): add very basic category theory
feat(library/data) add vectors and elimination of false to type
feat(library/data/category.lean)
feat(library/data/category.lean): more experimenting with local instances
feat(library/data/vector.lean) Add basic information about vectors
feat(library/data/vec.lean) add definition of length
feat(library/data/category.lean) Add some definitions for categories
feat(library/data/) Change vector and category to new conventions