Add support for neighbour calls.

[rbtcollins]

I haven't fixed up tests yet - wanted to put it out there for feedback.

[rbtcollins]

26	pnet_macros::register(&mut registry);	^^^^^^^^^^^^^ expected struct syntex::Registry, found a different struct syntex::Registry

= note: expected type &mut syntex::Registry = note: found type &mut syntex::Registry note: Perhaps two different versions of crate syntex are being used? --> build.rs:26:31 | 26 | pnet_macros::register(&mut registry); | ^^^^^^^^^^^^^

This is present in trunk I think.

[polachok]

I think it's fixed in master, try rebasing

[rbtcollins]

Yup, is happy now.

[polachok]

Thanks!