Generate Hasher artifact upon compile, not migrate

To fix intermittent `truffle test` failures, use Truffle's external
compiler system for generating a valid artifact for Hasher at
compile-time instead of in migrations/2_deploy_hasher.js.

- Define compileHelper.js script, which outputs temporary artifact
  data to `./build/Hasher.json`.

- Configure `external` compiler in truffle-config to run compileHelper
  and process result

- Remove usage of @truffle/artifactor in migration, since the artifact
  will now already exist

truffle-config.js

@@ -95,6 +95,12 @@ module.exports = {
         },
         // evmVersion: "byzantium"
       }
+    },
+    external: {
+      command: 'node ./compileHasher.js',
+      targets: [{
+        path: './build/Hasher.json'
+      }]
     }
   }
 }