simple transcription of Eckmann-Hilton argument (book 2.1.6)
Clean up conc^2-comm: move ⋆2=⋆'2 to PathGroupoid with correct proof, remove judgmental equality idp paths from ⋆2=conc^ and ⋆'2=conc^.
Merge pull request #8 from ystael/eckmann-hilton

simple transcription of Eckmann-Hilton argument (book 2.1.6)