Refactor tools to use a single CLI entrypoint
Instead of having separate binaries for the various tools, use a single CLI entrypoint "respdiff" and call the tools as subcommands, e.g.:
$ respdiff diff-answers ...
This makes it easier to distibute the tool and reduces the duplicity around CLI argument parsing.
In addition to the CLI changes, the tools have been renamed:
- msgdiff -> diff-answers
- transceiver -> transceive
clap was also updated to the latest version and its declarative facility was used for argument parsing.