add a CI job to verify msrv is still correct

.github/workflows/test.yml

@@ -38,3 +38,17 @@ jobs:
         with:
           command: test
           args: --verbose --all --no-default-features --features num
+  msrv:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-20.04, macos-10.15]
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+      - name: Install cargo-msrv
+        uses: baptiste0928/cargo-install@v1.1.0
+        with:
+          crate: cargo-msrv
+      - name: Verify MSRV
+        run: cargo msrv --verify