Bug fix for Mat.of_array_dyn/of_list_dyn

In the case of `of_list_dyn kind zero n []`, `n` is an arbitrary size,
but the older version accepted only zero as `n`.
Add tests for Mat.to_{array,list} and Mat.of_{array,list}_dyn
Add iterators on matrices: map, mapi, fold_{left,right,top,bottom} and fold_{left,right,top,bottom}i
Improve documentation
Add {Vec,Mat}.cnt, Vec.sub{cnt,dsc}vec_dyn and Mat.submat_dyn for usability
Remove a duplicated comment