use guile obarrays

use guile obarrays

* src/alloc.c (initialize_symbol): New function, extracted from
  `Fmake_symbol'.
  (Fmake_symbol): Use `scm_make_symbol'.
* src/emacs.c (main2): Define a symbol module.
* src/lisp.h (SYMBOLP, XSYMBOL): Update.
  (set_symbol_next): Remove. All callers changed.
* src/lread.c (obhash): New function.
  (Fintern, Fintern_soft, Funintern, oblookup, map_obarray): Use Guile
  obarrays.
  (init_obarray): Initialize a hash table of obarrays.