sb-win32: offer low-level bindings for file mapping functions

sb-win32: offer low-level bindings for file mapping functions

  Includes functions offered by Windows that are similar in
  spirit to mmap.

  Currently these are low-level FFI versions only, and no attempt
  is made to export them from sb-posix under the name mmap.  In
  sb-posix, only a wrapper for msync and definitions of several
  constants is offered, as needed for sb-simple-streams.

Thanks to Anton Kovalenko.

Also take this opportunity to sort the sb-win32 package
definition as a flat list.