[FIRRTL] Support recording/displaying deduplication results

Once deduplication is supported, it would be great if that pass could register (debug info?) which modules get deduplicated, and the size of each module (e.g. number of statements). This would help users figure out where they can most effectively reduce the duplication of modules.

Start by locating the FIRRTL deduplication pass and its existing support. Determine how the pass could register which modules were deduplicated and their sizes, then identify how those results should be displayed to users. Done means users can inspect deduplication results and module statement counts.

Written by the indexing model from the issue text.