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
