Define clippy allowed lints in code

[shssoichiro]

We can define e.g. #[allow(clippy::annoying_lint)] at the top of lib.rs and main.rs instead of specifying them inline in .travis.yml. This would make it easier for newcomers to the project to not need to know we have a special clippy command. This has been allowed for a few rust versions now (it used to break compiling if you weren't running clippy--now it's ignored if you don't have clippy and works fine). This would also remove the need for #1406.