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

migrations/2_deploy_hasher.js

@@ -1,21 +1,6 @@
 /* global artifacts */
-const path = require('path')
+const Hasher = artifacts.require('Hasher')
 
-const genContract = require('circomlib/src/mimcsponge_gencontract.js')
-const Artifactor = require('@truffle/artifactor')
-
-module.exports = function(deployer) {
-  return deployer.then( async () =>  {
-    const contractsDir = path.join(__dirname, '..', 'build/contracts')
-    let artifactor = new Artifactor(contractsDir)
-    let contractName = 'Hasher'
-    await artifactor.save({
-      contractName,
-      abi: genContract.abi,
-      unlinked_binary: genContract.createCode('mimcsponge', 220),
-    }).then(async () => {
-      const hasherContract = artifacts.require(contractName)
-      await deployer.deploy(hasherContract)
-    })
-  })
+module.exports = async function(deployer) {
+  await deployer.deploy(Hasher)
 }