Changes for Agda 2.3.4.
Use concrete fundamental groups.
Make it type check (in reasonable time) again.
Updates to covering spaces.
More support of alternative concatenation.
PathSeq is fine under 2.3.4.
Include results of covering spaces in index.agda.
Cover morphisms.
Better (and more correct) cover morphisms.
Use Ptd in CoverClassification.agda.
Exposing arguments is an equivalence.