ci/Cargo.toml: Specify and test MSRV.

The MSRV was specified in the README.md, so we can also specify
it in the standard location and validate it via CI on Linux.

.github/workflows/ci.yml

@@ -76,6 +76,20 @@ jobs:
       - name: cargo test compile
         run: cargo test --all-features --no-run --target wasm32-unknown-unknown
 
+  test-msrv:
+    runs-on: ubuntu-latest
+    name: cargo test msrv
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: install msrv toolchain
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: "1.65"
+
+      - name: cargo test
+        run: cargo test --all-features
+
   test-nightly:
     runs-on: ${{ matrix.os }}
     strategy:

Cargo.toml

@@ -4,6 +4,7 @@ version = "0.9.5"
 authors = ["Raph Levien <[email protected]>"]
 license = "MIT/Apache-2.0"
 edition = "2021"
+rust-version = "1.65" # When updating this, also update the README.md and CI.
 keywords = ["graphics", "curve", "curves", "bezier", "geometry"]
 repository = "https://github.com/linebender/kurbo"
 description = "A 2D curves library"