Split up CategoryTheory.jl into its own folder

[mehalter]

This should probably go in a new folder like /src/category/cateogy.jl if it keeps growing we could do

/src/category/category.jl
/src/category/span.jl
/src/category/cospan.jl
/src/category/decorated.jl

It is probably overkill to use 4 files for this right now, but that should be on the roadmap as the amount of CT code we write grows.

Originally posted by @jpfairbanks in #213 (comment)

[jpfairbanks]

We should think about migrating this code to the Catlab Standard Library. Basically for every doctrine C, we can generate an instance of Cospan(C). Decorated cospans should be built on top of that interface.