Switch from deep clone to copy-on-write for temp

In Stripe's deployment, we noticed that temp deep clones would take up
to 150ms. Aggregated over many tests, this could be very
expensive. Rather than have configuration temping be a known-expensive
operation, it seemed better to improve temp's performance.

One note is that this copy-on-write implementation will no longer
clone *values*, which is a slight backwards incompatibility. You
really shouldn't be mutating your configatron values directly though.