Minimize code repetition between PRX and PRD/PRS

[dressupgeekout]

PRX on one hand, and PRD/PRS on the other, are extremely similar file formats. There's got to be a way to minimize the differences in their implementations...

[dressupgeekout]

I have a few ideas of how to do this.

Consider the following:

$ prx -t <file>
$ prx -x -a <file>

Instead of assuming <file> is a single PRX, we can require the user to indicate whether it's a single PRX, OR a PRD/PRS pair. Perhaps something like:

$ prx -t --prx <file>
$ prx -t --prd <file1> --prs <file2>

Just thinking out loud here, mostly. Still gotta deduplicate the code underneath the hood, though. I might do something like I did with the recent MMFW work.