Stable Hash: Add feature, adjust CI, modify build.rs, add new API for accessing and setting the ahash seed in config.rs, make config.rs public

.github/workflows/build.yml

@@ -89,6 +89,37 @@ jobs:
         command: test
         args: ${{matrix.flags}}
 
+  nightly_features_build:
+    name: NightlyFeatureBuild
+    runs-on: ${{matrix.os}}
+    continue-on-error: ${{matrix.experimental}}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        flags:
+          - "--features stable_hash"
+        toolchain: [ nightly ]
+        experimental: [ false ]
+        include:
+          # smoketests for different toolchains
+          - { toolchain: nightly, os: windows-latest, experimental: false, flags: "" }
+          - { toolchain: nightly, os: macos-latest, experimental: false, flags: "" }
+      fail-fast: false
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{matrix.toolchain}}
+          override: true
+      - name: Test
+        uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: ${{matrix.flags}}
+
+
   # no-std builds are a bit more extensive to test
   no_std_build:
     name: NoStdBuild