Explore variadic map_rect

[WardBrian]

This was discussed by @SteveBronder and @wds15 and I at StanCon. It would be nice to have, but requires some serialization to move between MPI ranks. The compiler could potentially generate these functions, similar to how it currently does parameter reads?

[wds15]

This would be super cool to have! We should probably name this variadiac map_rect simply „map“…since there is no more rectangular thing in here.

The compiler is not something I work on, but let me know if I can help anyways.