[Sim] Add output-stream support and introduce sim.get_file

[nanjo712]

This PR added sim.get_file operation, and extended the sim.print/sim.proc.print operations to support control over the output stream.

This PR was split from #10146 .

[nanjo712]

Should we add an operation that directly represents stdout/stderr?

However, since the stream parameter is currently optional, it might be feasible to have sim.print default to stderr when no stream is specified.

If we need to do this, I think we might need to update the description of the print operation here to account for this detail.

[fzi-hielscher]

Thanks for breaking this out. I wasn't aware until now that FIRRTL allows dynamic filenames. That does make thinks a bit more complicated.

Should we add an operation that directly represents stdout/stderr?

However, since the stream parameter is currently optional, it might be feasible to have sim.print default to stderr when no stream is specified.

Yes, let's add them in a followup PR. The FIRRTL behavior of defaulting to stderr is kind of unusual and that should be stated explicitly in the IR. If no stream is specified we can just let the backend pick its default output stream.

[uenoku]

Thank you for breaking up the PR, I really appreciate. Also thank you @fzi-hielscher for thorough reviews.

[fzi-hielscher]

Thanks, only a small nit left.