Add CI check for msrv

.github/workflows/rust.yml

@@ -37,3 +37,19 @@ jobs:
     - uses: actions/checkout@v3
     - name: doc
       run: cargo doc --all-features
+  msrv:
+    runs-on: ubuntu-latest
+    # we use a matrix here just because env can't be used in job names
+    # https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability
+    strategy:
+      matrix:
+        msrv: [1.56.1]
+    name: ubuntu / ${{ matrix.msrv }}
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install ${{ matrix.msrv }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.msrv }}
+      - name: cargo +${{ matrix.msrv }} check
+        run: cargo check --all-features