Enable clippy on CI

Add a clippy configuration file configuring the MSRV. Add a github
actions job to run clippy on CI.

Please note the job does _not_ use `--all-targets` because doing so
causes:
```
error[E0554]: `#![feature]` may not be used on the stable release channel
--> src/lib.rs:46:54
|
46 | #![cfg_attr(all(test, feature = "unstable"), feature(test))]
```

Author: tcharding

.github/workflows/rust.yml

@@ -107,3 +107,18 @@ jobs:
           CARGO_TARGET_THUMBV7M_NONE_EABI_RUNNER: "qemu-system-arm -cpu cortex-m3 -machine mps2-an385 -nographic -semihosting-config enable=on,target=native -kernel"
         run: cd embedded && cargo run --target thumbv7m-none-eabi
 
+  Clippy:
+    name: Clippy
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - run: rustup component add clippy
+      - uses: actions-rs/cargo@v1
+        with:
+          command: clippy
+          args: --all-features -- -D warnings