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 branch 'eckmann-hilton' into book

Conflicts:
	lib/types/LoopSpace.agda
Book through 2.3.1