Change make_function to allow for multiple vector input arguments

[brianguenter]

make_function allows you to do this

make_function(f,x,y,z) where x,y,z are vectors of input vectors. But the runtime generated function returned by make_function expects a single vector argument which is the concatenation of x,y,z. 

This would make using the runtime generated functions simpler when there are multiple input vectors.

[gdalle]

This would also be nice for vector-jacobian and jacobian-vector products in terms of efficiency.
I ran into this with DifferentiationInterface.jl: whenever I want to call the generated function for a JVP, I have to feed it vcat(x, v), which leads to an allocation

[gdalle]

In concrete terms, solving this issue would allow me to remove the allocation due to vcat in these lines:
https://github.com/gdalle/DifferentiationInterface.jl/blob/3bfcd1f2dc005936c8defe7ca335953f20401713/ext/DifferentiationInterfaceFastDifferentiationExt/allocating.jl#L18-L29

[brianguenter]

@gdalle do you really want x,y,z to each be a vector of vectors? Or do you want each of them to be a vector of scalar input values?

[gdalle]

My bad, each being a vector of numbers is fine. Matrices would be ideal but I can handle the flattening on my end