defined the category of idempotents of a given category X
Merge branch 'master' of https://github.com/jmchapman/restriction-categories
partial optimization
	modified:   Categories.agda
	renamed:    RestrictionCat.agda -> MonicClasses.agda
	modified:   PartialMaps.agda
	new file:   PastingLemmas.agda
	modified:   Pullbacks.agda
	modified:   RestrictionCat.agda
	modified:   RestrictionPar.agda