Adding cargo semver checks (#369)

Adding a check that will run `cargo semver-checks` against each PR. 

This implements a check for PRs that don't bump the crate version. The
check will be run between the `HEAD` of the PR and `main`. So any
changes introduced in the PR vs main that break semver are picked up,
and the PR will be labeled with `breaking-change`.

Currently can't test the actual adding of a label because (probably) the
check needs to be in the main repo, rather than in a PR branch, to be
allowed to get write access to the repo.

So sadly to actually test this we'll need to merge and see if it works.
(hooray github actions dev workflow)

.github/workflows/semver-checks.yml

@@ -0,0 +1,43 @@
+name: semver-checks
+
+on: [pull_request]
+
+env:
+  CARGO_TERM_COLOR: always
+  RUST_BACKTRACE: 1
+
+jobs:
+  check_if_pr_breaks_semver:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+        name: checkout full rep
+        with:
+          fetch-depth: 0
+      - name: Install minimal stable
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: default
+          toolchain: stable
+          override: true
+      - uses: Swatinem/rust-cache@v2
+      - name: Install cargo-semver-checks
+        shell: bash
+        run: |
+          cargo install cargo-semver-checks --locked
+      - name: Run check
+        shell: bash
+        run: |
+          cargo semver-checks --all-features --baseline-rev ${{ github.event.pull_request.base.sha }}
+      - name: On Failure
+        if: failure()
+        uses: actions-ecosystem/action-add-labels@v1
+        with:
+          labels: breaking-change
+      - name: On Success
+        if: success()
+        shell: bash
+        run: |
+          echo "Checks succeed"