Add more isomorphisms

Specifically:
 * Congruence over products and sums
 * Congruence over Maybe
 * Demonstrate isos of Maybe using sum and unit isos
Weakening and shifting of Fins

Add two functions:
 * weakenN weakens a Fin n m times
 * shift adds a Nat to a Fin, extending the bound
Add isomorphisms for Fin
Make spacing consistent
Merge pull request #748 from david-christiansen/feature/isomorphisms

Add more isomorphisms