Multicolumn indexes

Multicolumn indexes are useful in databases and also would be useful for ACSets. For example, a mapping table (i.e. a span `A<-R->B`, encoding a relation) is naturally indexed by its two outgoing morphisms to quickly check if `(a,b)` are related.

I'm not sure what needs to change in the underlying data structure, but an interface might look like this:

```
@present SchTest(FreeSchema) begin
(A,B,C,R)::Ob
f::Hom(R,A)
g::Hom(R,B)
h::Hom(R,C)
end

@acset_type Test(SchTest, index=[:h, (:f,:g)])

X = apex(terminal(Test))

incident(X, (1,1), (:f,:g)) # == [1]
```