Port changes from terence/geb-dev-15

[rokopt]

A number of changes made parallel to the directory structure change:

• Code-factoring and further development of Poly(FinSet)
• Dirichlet functors and natural transformations with vertical-Cartesian factoring
• Some equivalent base-function formulations of algebras/coalgebras
• Factoring out some notions of equalizers and Kan extensions in the Idris type system
• Definition of dependent polynomial endofunctors, including in refined-type form
• More implementation of delimited continuations
• More implementation and explanation of codensity monad