doc(make/msys2): instructions for building native windows Lean binary
doc(README.md): group build instructions by system
fix(.travis.yml): trying old configuration for TESTCOV
feat(library): add constructions of categories, some changes in eq, sigma and path

in eq, add theorem for proof irrelevance and congruence for binary functions
in sigma, add some support for triplets
in path, comment out two unneccesary definitions
in category, add Cat, slice, coslice, product and arrow categories, also add fully bundled approach
feat(library/data/int): remove duplicate theorem, fix one sorry
feat(library): changes in eq.lean sigma.lean and category.lean

in eq.lean, make rec_on depend on the proof, and add congruence theorems for n-ary functions with n between 2 and 5
in sigma.lean, finish proving equality of triples
in category.lean, define the functor category, and make the proofs of the arrow and slice categories easier for the elaborator