This further cleans up the prime-generation code, to the point where
the main primegen() function has almost nothing in it. Also now I'll
be able to reuse M-R as a primitive in more sophisticated alternatives
to primegen().
This reverts commit a7bdefb394.
I had accidentally mashed it together with another commit. I did
actually want to push both of them, but I'd rather push them
separately! So I'm backing out the combined blob, and I'll re-push
them with their proper comments and explanations.
The old API was one of those horrible things I used to do when I was
young and foolish, in which you have just one function, and indicate
which of lots of things it's doing by passing in flags. It was crying
out to be replaced with a vtable.
While I'm at it, I've reworked the code on the Windows side that
decides what to do with the progress bar, so that it's based on
actually justifiable estimates of probability rather than magic
integer constants.
Since computers are generally faster now than they were at the start
of this project, I've also decided there's no longer any point in
making the fixed final part of RSA key generation bother to report
progress at all. So the progress bars are now only for the variable
part, i.e. the actual prime generations.
