Add _==_ notation for identity type
Add SetEndofunctor
Add SetMonad
Add Multisetoid
Minor cleanup
Whoops!
Add forgotten composition law
Implement List monad
Rename Multisetoid => GeneralizedMultisetoid; add Multisetoid
Cleanup in Prelude (details below)

- Remove unused is-prop
- Make everything here universe-polymorphic (it will not affect the use
  of this machinery in code that has type-in-type turned on).
Add Σ types to prelude
Add notation for Σ
Add reflexivity and transivitity to T-Multisetoid; thx @darinmorrison
Update transitivity definition
Remove unnecessary parentheses
Rename _List => _list
Add composition operator _∘_
Formulate T-multisetoid s.t. T is container; add symmetry

TODO: give the functor for container extension once and for all
TODO: give the list container and its monad
Add kit I'll need later for giving the list container
Give the functor for the container's extension