til_args: introduce --print-module-contexts

Some rudimentary instrumentation for monitoring the active module
contexts alongside the pipes

You probably want to redirect stderr to a file when using
--print-pipes and/or --print-module-contexts...

e.g.
```
rototiller --defaults --go --print-pipes --print-module-contexts 2>/dev/null
```

or, if you still want to monitor FPS or log_channels=on in rtv,
2>/file/to/tail then tail -F /file/to/tail in another terminal.

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