Make a CLI tool for diagnosing payload errors

Tool would take AMPHTML on stdin, run parse-n-print (`Process` with no transformers) twice in series, and then return nonzero status if `PnP(doc) != PnP(PnP(doc))`. Optionally, output some sort of human-readable diff.

Start by locating the parse-n-print implementation and the Process entry point, including how AMPHTML input is read and status codes are returned. Verify the tool runs the no-transformer process twice in series, compares the first and second results, returns nonzero on a mismatch, and determine the scope of any human-readable diff.

Written by the indexing model from the issue text.