Process reorganisation: the System.Process library moves into base,
and System.Cmd is re-implemented in terms of it.

Thanks to Krasimir Angelov, we have a version of System.Process that
doesn't rely on the unix or Win32 libraries.  Normally using
unix/Win32 would be the right thing, but since we want to implement
System.Cmd on top of this, and GHC uses System.Cmd, we can't introduce
a bunch of .hsc dependencies into GHC's bootstrap libraries.

So, the new version is larger, but has fewer dependencies.  I imagine
it shouldn't be too hard to port to other compilers.