use vectors for symbol slots

use vectors for symbol slots

* src/alloc.c (initialize_symbol): Use Guile vectors to store symbol
  slots.

* src/lisp.h (lisp_h_SET_SYMBOL_VAL, lisp_h_SYMBOL_CONSTANT_P)
  (lisp_h_SYMBOL_VAL): Use Guile vectors to store symbol slots.
  (sym_t): New typedef.
  (SYMBOL_SELF, SET_SYMBOL_SELF, SYMBOL_REDIRECT, SET_SYMBOL_REDIRECT)
  (SYMBOL_CONSTANT, SET_SYMBOL_CONSTANT, SYMBOL_DECLARED_SPECIAL)
  (SET_SYMBOL_DECLARED_SPECIAL): New macros.
  (struct Lisp_Symbol): Change all field names. All references changed
  to use `sym_t' and the above macros instead.