{#addrof <path> #} retrieves the address of a field of a c
structure, following <path>. It behaves much like get or set, but
instead of marshalling the target it returns a pointer to it.
This is useful e.g. when the associated Haskell type doesn't
have a Storable instance, for example SockAddr.
See also: http://www.haskell.org/pipermail/c2hs/2013-March/001113.html
