Replace run-time on-demand initialisation of PL_bitcount with a constant table.

Replace run-time on-demand initialisation of PL_bitcount with a constant table.
(The table is 256 bytes; the run-time initialisation code is larger than this!)
Adapt generate_uudmap.c to generate the initalisation block for PL_bitcount,
writing the code to bitcount.h, using the same approach as uudmap.h.

To preserve binary compatibility:
for MULTIPLICITY:
  keep Ibitcount in the interpreter structure, but remove all the macros that
  access it. PL_bitcount is a new symbol in the object file, which won't clash
  with anything as that name wasn't used before.
otherwise:
  keep PL_bitcount as a char *, but initialise it at compile time to a new
  constant array PL_bitcount array. Remove the code that attempts to Safefree()
  it at interpreter destruction time.