Shouldn't assume containers have cbegin()

[eyalroz]

The span container doesn't have a .cbegin() metho in all C++ versions; and specifically - our cuda::span doesn't. And yet, our code in program.hpp, add_headers(), line 366 - we assume it exists,. We need to drop that assumption, use begin() and end()`, and burden the user with ensuring the range/container they provide lets us use that.