upgrading nom to version 7.

[plugwash]

Hi, I've been working on rust packaging in Debian and have noticed that the latest version of this crate depends on an older version of nom. While we do have a mechanism for packaging multiple versions of crates we try to avoid it where possible.

I tried patching the nom dependency to version 7 but got a whole bunch of errors. Specifically the most obvious issue seems to be the dropping of macros.