test(tests/lean/run): add section test
fix(frontends/lean): name clash inside section, fixes #181
feat(library/data/category.lean) the definition of category doesn't depend on 'mor' anymore; make iso a class; add theorems
fix(library/data/category): minor problem that was being masked by bug #182, fixes #183
feat(frontends/lean): add proof-qed expression

Remark: we still have to add support to it in the elaborator.
Right now, it is just an embellished parenthesis.
fix(library/data/category): mark definitions as abbreviations
feat(library/data/category): add composition of functors