Implement category of sketches

[kris-brown]

https://ncatlab.org/nlab/show/sketch "The category of sketches is well behaved: it is complete, cocomplete, cartesian closed and has a second symmetric monoidal closed structure."

Sketches are quite unwieldy to work with directly, though they'd much easier to work with as (co)limits of simpler building block sketches. We can also take advantage of this higher structure when enumerating models (#2)

[kris-brown]

progress started in this branch

sketch morphisms are at least as difficult as FinFunctors (the case where there are no (co)cones). We might want to leverage the work in this Catlab PR with limits and colimits for FinCats.