- Add `process_ipv6` to `EthernetInterface`
- Add basic test for `process_ipv6`
- Add `deny(unused)` if either proto-ipv4 or proto-ipv6 is enabled
- Add `cfg`s where needed to avoid compile time errors due to the above
- There are several warnings that are thrown when running `cargo doc`. Fix
these warnings.
- Convert all module documentation to use /*! for consistency.
The rate of emission of neighbor discovery packets is already
limited at the level of the entire neighbor cache, but poll()
would uselessly spin until the answer arrives (if ever).