Split build job into separate jobs per feature configuration

GitHub will run up to 20 jobs in parallel, so can get faster builds by
running jobs in parallel. With the extensive caching we use, this
should result in about the same load for GitHub.

Together with #399, this brings the time for running all tests down to
1.5 minutes, removing about 2 minutes from every PR.

.github/workflows/build.yml

@@ -24,6 +24,13 @@ jobs:
         rust:
           - stable
           - nightly
+        features:
+          - name: no default features
+            flag: --no-default-features
+          - name: all features
+            flag: --all-features
+          - name: default features
+            flag: ''
 
     steps:
       - name: Checkout repository
@@ -34,14 +41,8 @@ jobs:
 
       - uses: Swatinem/rust-cache@v1
 
-      - name: Test with default features
-        run: cargo test
-
-      - name: Test without default features
-        run: cargo test --no-default-features
-
-      - name: Test with all features
-        run: cargo test --all-features
+      - name: Test with ${{ matrix.features.name }}
+        run: cargo test ${{ matrix.features.flag }}
 
   windows:
     name: Build on Windows (stable)