github/actions: Add a workflow to build

Since we're eyeing at a Windows build, we should also have some CI
system build on Windows, so that developers that do not run it can
access build logs.

Most of the builds currently fail, but that's okay, the intention is
that they become more green over time.

If you are ever spammed by notifications about these on GitHub, disable
these at https://github.com/settings/notifications (scroll to "System"
and the row "Actions").

.github/workflows/build.yml

@@ -0,0 +1,32 @@
+name: build
+on:
+  push: { branches: [ '*' ] }
+  workflow_dispatch:
+concurrency:
+  group: ${{ github.sha }}
+  cancel-in-progress: true
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+         - macos-latest       # arm64
+         - ubuntu-latest-arm  # arm64
+         - ubuntu-latest      # x64
+         - windows-latest-arm # arm64
+         - windows-latest     # x64
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: actions/cache@v4
+        with:
+          path: ~/.cargo
+          key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-home
+      - uses: actions/cache@v4
+        with:
+          path: target
+          key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-target
+      - run: cargo build --workspace --verbose --all-features
+      - run: cargo test  --workspace --verbose --all-features