Initially I had hoped that the deprecated `#![allow(clippy)]` would no
longer be put into the generated rust files, but -- as of 2019-01-30 --
it still is (see [1] for details). Since we explicitly update the
protobuf files I decided to *manually edit the generated code* and
replace this with `#![allow(clippy:all)]`. Hopefully, by the time we do
the next upgrade, no such manual tweaking would be necessary anymore. I
think the benefit of a less polluted clippy output is worth it this
time.
[1]: https://github.com/stepancheg/rust-protobuf/pull/332
