Set up CodeCov, fix formatting and clippy warnings on stable Rust (#19)

* add codecov token

* fix tendermint dep

* add token to repo

* fix fmt

* Fix clippy warning

* Switch back CI to stable Rust

* Adapt rustfmt config for stable

Co-authored-by: Romain Ruetschi <romain@informal.systems>

.github/workflows/rust.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           override: true
       - uses: actions-rs/cargo@v1
         with:
@@ -28,7 +28,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           components: clippy
           override: true
       - uses: actions-rs/clippy-check@v1
@@ -42,7 +42,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           override: true
       - uses: actions-rs/cargo@v1
         with: