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

compileHasher.js

@@ -0,0 +1,21 @@
+// Generates Hasher artifact at compile-time using Truffle's external compiler
+// mechanism
+const path = require('path')
+const fs = require('fs')
+const genContract = require('circomlib/src/mimcsponge_gencontract.js')
+
+// where Truffle will expect to find the results of the external compiler
+// command
+const outputPath = path.join(__dirname, 'build', 'Hasher.json')
+
+function main () {
+  const contract = {
+    contractName: 'Hasher',
+    abi: genContract.abi,
+    bytecode: genContract.createCode('mimcsponge', 220)
+  }
+
+  fs.writeFileSync(outputPath, JSON.stringify(contract))
+}
+
+main()