Create an aya_deref! macro to facilitate long chains of raw pointer dereferences

[willfindlay]

As per the discussion in #71, it would be beneficial to have some kind of an aya_deref! macro to make long chains of raw pointer dereferences more ergonomic (e.g. you can imagine something like aya_deref!(file, f_inode, i_ino) to get the inode number from a file struct). You can imagine that the semantics could be similar to BPF_CORE_READ and friends from libbpf (although with a slightly different purpose).

I'm filing this tracking issue so we have somewhere to discuss possible options and maybe get started on a draft PR soon.