Add SemVer compatibility checks to CI

Even small changes to item visibility during fixes might introduce
SemVer violations (see
https://doc.rust-lang.org/cargo/reference/semver.html#change-categories
for a list of changs that would be considered major/minor).

To make sure we don't accidentally introduce such changes, we here add a
new semver-checks CI job that utilizes cargo-semver-checks
(https://github.com/obi1kenobi/cargo-semver-checks), and have it run on
any push or pull requests.

.github/workflows/semver.yml

@@ -0,0 +1,15 @@
+name: SemVer checks
+on: [push, pull_request]
+
+jobs:
+  semver-checks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v4
+      - name: Install Rust stable toolchain
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain stable
+          rustup override set stable
+      - name: Check SemVer
+        uses: obi1kenobi/cargo-semver-checks-action@v2