Higher Order Functions on ACSets

We should implement the higher-order functions that are easy to do so on ACSets.

```julia
function map(f::FunctionWrapper{S,T}, data::Symbol, acs::ACSet{CD,AD,Ts})
```

This would return an ACSet where the `Data` type corresponding to `data` in Ts (which should be `S`) is replaced by `T`. I wish there was a way to express this in types, but I'm not sure if it's possible... Best approximation would be

```julia
function map1(f:FunctionWrapper{S,T}, acs::ACSet{CD,AD,Tuple{T}})::ACSet{CD,AD,Tuple{S}}
```

We should also have `filter`, which takes in a function of a "part", and all the parts that return true are copied to a new array, along with some other parts? The categorical semantics of this need to be worked out a bit.

And then `filterMap`, of course

I don't know if any type of `fold` makes sense, but if so, we should have it. At some point, I'm just going to go through a checklist of all the functions in dplyr, and figure out which ones make sense for ACSets.

And then also maybe some sorting functions?