Adding haskell.nix based default and shell.nix (#141)

Co-authored-by: Ben Ford <ben@perurbis.com>

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: "Nix CI"
+on:
+  pull_request:
+  push:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2.3.1
+    - uses: cachix/install-nix-action@v10
+      with:
+        skip_adding_nixpkgs_channel: true
+    - uses: cachix/cachix-action@v6
+      with:
+        name: cdodev
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - run: cachix use iohk && cachix use cdodev
+    - run: nix-build