Represent RAM in banks.  This touches everything (it hasn't all been tested!)
Always require libspectrum to init successfully

We may want to use its machine capabilities later.  In the meantime, the
 handling of 'disable tape/snap loading if libspectrum fails' was broken.
Beginnings of 128k support
Rest of paging - 128k support now works!  (Except for timings, debugger, etc.)