0.9.5.51:

0.9.5.51:
        Start merging some safe and uncontroversial changes from from
        gencgc-pagetable-branch. Hopefully once all of the safe changes
        has been merged, taking care of the remaining changes will be
        less daunting.

        Cleanups, should cause no functional changes.

        * Add typedefs for indices into the generation/page tables for
          clarity and to ease changing their sizes later. Use these
          consistently in gencgc.c.
        * Change some suspicious 'unsigned's to 'unsigned long's.
        * Define logical names for the various special generations, instead
          of using NUM_GENERATIONS [- 1, + 0, + 1].
        * Rename update_x86_dynamic_space_free_pointer() to
          update_dynamic_space_free_pointer().