[CI] Automatically compile and package Windows binaries for releases

Let's try this and see if I like the result…
…and I do!

• While this action uses the `x86_64-pc-windows-gnu` triple instead of
  `x86_64-pc-windows-msvc` and is therefore slightly less native with
  the additional dependency on MSVCRT.DLL, it's only 44 KiB larger,
  which is more than offset by specifying `codegen-units = 1`.

• *Any* type of CI build avoids the inherent path leaking problem with
  PDBs, without requiring any of the crazy and nightly-exclusive flags
  documented at https://github.com/johnthagen/min-sized-rust,

Part of P0268, funded by Ember2528.

.github/workflows/release.yml

@@ -0,0 +1,22 @@
+on:
+  release:
+    types: [created]
+
+jobs:
+  release:
+    permissions: write-all
+    name: release ${{ matrix.target }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [x86_64-pc-windows-gnu]
+    steps:
+      - uses: actions/checkout@master
+      - name: Compile and release
+        uses: rust-build/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          RUSTTARGET: ${{ matrix.target }}
+          EXTRA_FILES: "COPYING README.md"