CI: Run our fuzz targets on our corpora

This doesn't have libFuzzer generate new inputs in CI, only verifies that we can
still successfully process our existing seed inputs in each of our targets'
corpora.

.github/workflows/main.yml

@@ -62,6 +62,30 @@ jobs:
         name: doc-api
         path: target/doc
 
+  # Download our libFuzzer corpus and make sure that we can still handle all the
+  # inputs.
+  fuzz_corpora:
+    name: Fuzz Corpora
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/checkout@v1
+      with:
+        repository: bytecodealliance/wasmtime-libfuzzer-corpus
+        path: ./wasmtime/fuzz/corpus
+        ref: refs/heads/master
+    - uses: ./.github/actions/install-rust
+      with:
+        toolchain: nightly
+    - run: cargo install cargo-fuzz
+    - run: cargo fetch
+      working-directory: ./fuzz
+    # NB: the `-runs=0` means that libFuzzer won't generate new inputs, only run
+    # the seeds from the corpus.
+    - run: cargo fuzz run compile -- -runs=0
+    - run: cargo fuzz run instantiate -- -runs=0
+    - run: cargo fuzz run instantiate_translated -- -runs=0
+
   # Perform all tests (debug mode) for `wasmtime`. This runs stable/beta/nightly
   # channels of Rust as well as macOS/Linux/Windows.
   test: