adding to restriction a lint that check for multiple inherent implementations

[mockersf]

for #414

warn on each subsequent implementation of a struct, with a note showing the first implementation

[oli-obk]

I'd rather have this in the restriction group. It's something that you don't generally want in my book, but can be very helpful for satisfying standards or company policy

[mockersf]

oh well latest nightly is not happy with clippy 😢
FF #![feature(macro_at_most_once_rep)] is needed as macros declare_lint and lint_array were changed
then there are a lot of errors like no field 'name' on type '&rustc::hir::Field' because of another change

[mati865]

Nightly breakage is handled in #2817

[phansch]

retriggering travis

[mati865]

@phansch It needs to be rebased on top of master branch to make Travis pass.

[Manishearth]

looks like you cherry picked the rustup? Please rebase over master instead 😄

[mockersf]

my bad, my git-fu is not that good

[mockersf]

one of the integration build failed with

error: component download failed for rust-docs-x86_64-unknown-linux-gnu
info: caused by: could not download file from 'https://static.rust-lang.org/dist/2018-05-29/rust-docs-nightly-x86_64-unknown-linux-gnu.tar.xz' to '/home/travis/.rustup/downloads/dafb4b05358273e2b1ce3e290a5d3a3a5ddde75ec3cb9431c028c3e1e4f7cea4.partial'
info: caused by: http request returned an unsuccessful status code: 503