Add 32-bit cross testing to CI

Miri makes cross testing extremely easy. Bonus: We get Miri testing,
though that is not super important, as we don't use any unsafe anyways.

.github/workflows/rust.yml

@@ -1,13 +1,19 @@
 name: Rust
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
     runs-on: ${{ matrix.os }}-latest
     steps:
     - uses: actions/checkout@v2
     - uses: rust-lang/simpleinfra/github-actions/simple-ci@master
+    - name: "32-bit cross testing"
+      run: |
+        rustup toolchain install nightly
+        rustup override set nightly
+        rustup component add miri
+        cargo +nightly miri test --target i686-unknown-linux-gnu
     strategy:
       matrix:
         os: [ubuntu, windows, macos]