Make --output-prefix optional (or remove it entirely)

[randomPoison]

Currently the --output-prefix flag on the rewriter is required to be specified. I'm not sure why we need that prefix to be specified by the user, it seems like we could use a default prefix of ia2_ or wrapper_ (or something similar). If there's a reason why we need to give the user an option to control that prefix, then I think we should make --output-prefix optional, though I think the better solution would be to remove it entirely if that wouldn't cause any problems.

[kkysen]

I agree. Having this option doesn't seem very useful, and it also makes referring to these files in documentation, etc. more confusing. It should also go in --output-directory (I forgot if it already does, since I just pass an absolute path).

[randomPoison]

It currently puts those files in whatever directory you run the rewriter in. I agree they should go in the output directory, but when I brought that up the other day I think @ayrtonm said that doesn't make sense for some reason (I don't remember what exactly).