Add clippy.toml

This tells clippy to only suggest lints
which are possible with the current MSRV

Also do use the stable clippy toolchain
instead of nightly.

Author: Sh3Rm4n

.clippy.toml

@@ -0,0 +1 @@
+msrv = "1.50"

.github/workflows/ci.yml

@@ -103,7 +103,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           target: thumbv7em-none-eabihf
           override: true
           profile: minimal