* read.c (scm_init_read): intitialise scm_read_hash_procedures

	* read.c (scm_init_read): intitialise scm_read_hash_procedures
	(idea from Mikael: make it a pair so scm_permanent object only
	called once.)
	(scm_read_hash_extend): don't call scm_permanent_object.
	(ideas from Mikael): if chr is already in the list, replace its
	procedure instead of appending it again.  If chr is #f, remove
	it from the list.
	(scm_get_hash_procedure): take CDR of scm_read_hash_procedures.