Move program interface wrappers to a new dpctl.program submodule

The SyclProgram class is really very specific to Numba and does not really belong under dpctl. Although, having the ability create a kernel from source like CuPy can be useful. For now, let us move it out of `sycl_core.pyx` and directly inside `dpctl` into a sub-module called `dpctl.program` module.