support tracing of the plugin's conversion

[zliu41]

There is a change up with a crude version of this, but it needs to be more comprehensive and more controllable

a number of improvements we should make:

• control these via plugin flags (already done)
• convert to general levels and/or categories
• distinguish between plugin-user debugging (what instance am I missing?) and plugin-dev debugging (why didn't it find the instance that exists?)
• optionally only output on a failure
• replace trace with something Writer-y (which also makes output-on- failure feasible)
• take advantage of trampolining to do this in a single place
• trampolining (which we already need for the conversion) gives us a middle ground between an opaque fold and recursion-scheme level introspection.