Code the 2-sphere directly
Make some connections to the book in Chapter 6
Put the 2-sphere in Set

It was in Prop.
Merge pull request #337 from JasonGross/2-sphere-direct

2 sphere direct.
Merging as this has been waiting for two weeks and is not in the critical part.
Add two more kinds of duals

The identifiers L and R refer to which ^op is removed/inverted.
Add generalized version of identity functor

Necessary to get between two identical functor categories which use
different instances of funext.
Functorialize yoneda
Clean up and unify Yoneda a lot

Unfortunately, it seems that the [Defined] at the end runs forever.