to_matrix and to_vector to apply to general containers

[edit: generalized for vectors by @bob-carpenter]

Add this signature for to_matrix:

array[N_cols] vector[N_rows] arr_Vec ;
matrix[N_rows,N_cols] mat_from_arr_Vec = to_matrix(arr_Vec) ;//proposed handling of new signature

and general signatures for to_vector for any container:

vector[N] to_vector(some container);

The reason to do this is to make it easy to reshape efficiently for likelihood and prior density increments.

This already works:

array[N_rows] row_vector[N_cols] arr_Rvec ;
matrix[N_rows,N_cols] mat_from_arr_Rvec = to_matrix(arr_Rvec) ;

I've already started a branch of stanc3 exposing this method, but presumably it has to exist in the first place.

Start by reviewing the existing to_matrix handling for array row_vectors and the generalized vector work described in the issue. Then inspect the referenced stanc3 branch to understand the expected signatures and container coverage. Done means the requested to_matrix and to_vector conversions are supported consistently and the issue's example cases work.