This implements the KISS suggested in https://github.com/serde-rs/serde/issues/790.
It is possible that one of the other approaches may be better but this
seemed like the simplest one to reignite som discussion.
Personally I find the original suggestion of adding two traits perhaps slightly
cleaner in theory but I think it ends up more complicated in the end
since the added traits also need to be duplicated to to the `Seed`
traits.
Closes#790
* move to cargo clippy instead of using the clippy plugin
fixes#729
* non-exectable scripts must be run with `sh`
* don't build serde in the clippy travis job
* only run clippy tests if installing clippy succeeds
* why is travis so picky?
* no more serde_codegen
* serde_test_suite_deps has no features
* don't use empty loops, llvm optimizes them to undefined behaviour
* abort the clippy job when clippy lints are triggered
* use caches on travis to speed up builds
* why are we even using `travis-cargo`?
* need to reinstall clippy frequently due to nightly updates
* command line tools are hard
Markus Westerlind