automate crate publishing

This introduces a new section of the GitHub Actions workflow which will
publish the crate upon a new tag being pushed. Note that this requires a
new project secret, `CRATES_TOKEN`.

.github/workflows/rust.yml

@@ -63,3 +63,16 @@ jobs:
       uses: coverallsapp/github-action@master
       with:
         github-token: ${{ secrets.GITHUB_TOKEN }}
+
+  publish-crate:
+    if: startsWith(github.ref, 'refs/tags/')
+    runs-on: ubuntu-latest
+    needs: [test]
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+      - uses: actions/checkout@v2
+      - name: Publish
+        shell: bash
+        run: |
+          cargo publish --token ${{ secrets.CRATES_TOKEN }}