Cleanup withdrawer code

As noted in #17 there are some spots where the code can be cleaned up / better optimized. This work should be done as a fast-follow.

Start by reading issue #17 and tracing the withdrawer utility's proving and finalizing paths. The issue does not name files, tests, or specific cleanup targets, so identify the referenced spots before making changes. Done means the agreed cleanup and optimization work is addressed and the existing test suite passes.

Written by the indexing model from the issue text.