Use updated `build_function`

[Vaibhavdixit02]

Looks like Symbolics has deprecated use of build_function(<:Array{Equation}) and needs an array of Nums(/Terms ?). We should update our use of it here.

[ChrisRackauckas]

Why would we do it on a Vector{Syms}? Seems odd?

[Vaibhavdixit02]

Oh I meant Array{Equation} https://github.com/JuliaSymbolics/Symbolics.jl/blob/5708f21726cefb0029b0900eafbf5bed23bbd266/src/build_function.jl#L458. And that's the deprecated one, not asking for it, just opened this to track that we update here

[ChrisRackauckas]

Oh yes, Array{Equation} was only ever implemented for the Stan and MATLAB ones, so they were kind of weird. We should be doing build_function on the .rhs. That needs to bubble into what happens automatically.