No type piracy in ACSet schema description types

[epatters]

Resolves #382.

[olynch]

So, the one thing is, and this is the reason I made these accessors in the first place, that I use ob, hom, data, and attr as variable names all over the place. This isn't necessarily a reason why this shouldn't be merged, but it's something to keep in mind, and maybe we should shift to using variable names C, f, X, and a.

[epatters]

One indication that this isn't too bad is that in the whole codebase, I only had to change the variable names in one or two places! You can't use ob as a variable name if you also use it as a function, but you can use it in list comprehension variables and local function argument names, which is often good enough.

[KristofferC]

An alternative is perhaps to create (singleton) instances of the types and then use getproperty on those (similar to how nothing tends to be used over Nothing). Using types as "values" tends to be a bit awkward, especially when you want to start adding methods to them.