message: Bring `codecov` back
  * Revert "Remove `codecov` CI stage (#1405)"
  This reverts commit
  * Set `RUSTC_BOOTSTRAP`
  * Update .gitlab-ci.yml
diff: Parity's ink! for writing smart contracts
  </h1>
  -[![linux][a1]][a2] [![loc][e1]][e2] [![stack-exchange][s1]][s2]
  +[![linux][a1]][a2] [![codecov][c1]][c2] [![coveralls][d1]][d2] [![loc][e1]][e2] [![stack-exchange][s1]][s2]
  [a1]: https://gitlab.parity.io/parity/ink/badges/master/pipeline.svg
  [a2]: https://gitlab.parity.io/parity/ink/pipelines?ref=master
  +[c1]: https://codecov.io/gh/paritytech/ink/branch/master/graph/badge.svg
  +[c2]: https://codecov.io/gh/paritytech/ink/branch/master
  +[d1]: https://coveralls.io/repos/github/paritytech/ink/badge.svg?branch=master
  +[d2]: https://coveralls.io/github/paritytech/ink?branch=master
  [e1]: https://tokei.rs/b1/github/paritytech/ink?category=code
  [e2]: https://github.com/Aaronepower/tokei#badges
  [f1]: https://img.shields.io/badge/click-blue.svg
  