Split scanner.c, move io part out (say into io.c and io.h)

[fingolfin]

I always found it very confusing that things like OpenInput, Pr etc. are inside scanner.c. Sure, I understand nowadays why (it is also explained in the comment at the top of scanner.c and scanner.h. I still find it confusing, and if I don't touch the GAP kernel for some time, will forget.

As it is, the two parts seem to be only very loosely coupled, so I think it'd be rather easy to separate them. Further separating the "scanner" / parser part from the IO part should also help with later refactoring.

The new files could be called io.c and io.h (although there is a slight chance of confusion with the io package, I guess). I actually would have suggested iostream.{c,h}, but those already exist -- and contain code to handle managing subprocesses (so perhaps that file should be renamed, too?)