feat(library/hott): port Floris' category implementation
feat(library/hott): try to replace the proof irrelevance
fix(library/hott): finish associativity proof
feat(library/hott): finish porting of natural_transformation.lean
feat(library/hott): port a part of algebra/category/constructions.lean, slice category still to do
feat(library/hott): add iso_of_path lemma for precategories
chore(library/hott): move precategory definition to its own folder
chore(library/algebra): change category to be a structure
fix(library/hott): adjust expliciteness of arguments
chore(library/hott): change precategory to structure, fix morphism.lean
chore(library/hott): make constructions.lean compile, still lots of work to do
feat(library/hott): add definition of category
chore(library/hott): turn isomorphic into structure
chore(library/hott): make precategory use the isomorphic structure
feat(library/hott): complete theorems about truncatedness of isomorphism sets
feat(library/hott): complete proof that object types of proper hott categories are one truncated
feat(library/hott): add proof that the type of nat trafos is a set

The characteriszation of nat trafo by sigma types up to equivalence is still to be done (two unsuccessful proof attempts included)
fix(library/hott): issues resulting from merge
fix(library/hott): issues resulting from merge