Introduce `ProgramHandle` abstraction

[danielocfb]

We should think about introducing a ProgramHandle type, similar to what we have with MapHandle. We seem to have a bunch of somewhat confusing APIs that work on file descriptors, when really I think they should be abstracted behind such a type.

[danielocfb]

While at it, let's also think about https://docs.rs/libbpf-rs/latest/libbpf_rs/struct.MapInfo.html and whether we want a ProgramInfo similar to it.