Patch stanfunctions support for future versions of Stanc3

[andrjohns]

This PR updates the compatibility of exporting standalone functions for more recent versions of stanc3 (i.e., the experimental rstan, or custom versions with the latest stanc3).

There are three main behaviours patched:

• Recent stanc3 will emit standalone function plain types using a forward declaration, whereas older versions using a functor within a struct
• Default template parameters under stanc3 2.31 cause a compiler error under clang (they use 0 instead of nullptr)
• There is no longer a stan/math/prim/mat/fun/Eigen.hpp file

[jgabry]

Great, thanks!