Clean up all use of the *-space-free-pointers.

Clean up all use of the *-space-free-pointers.

  * These are symbols whose value slots contain unboxed word-aligned
byte pointers.  Thus, they appear to lisp as fixnums.  They are not,
however, guaranteed to be word pointers.

  * Shift by n-fixnum-tag-bits instead of shifting by word-shift or
scaling by n-word-bytes in order to obtain byte pointers.