* * print.c (scm_iprin1, scm_prin1, scm_iprlist): Circular

*	* print.c (scm_iprin1, scm_prin1, scm_iprlist): Circular
	references now have a new appearance which is more compact and
	also gives a clue about what the target of the reference is.
	By setting parameters in the print state, more fancy printing can
	be achieved.  This is used by the (not yet commited) backtrace
	code.

	* print.c: Added #include "struct.h".  Removed function
	scm_prlist.

	* print.c (scm_prin1): Print states are now allocated when calling
 	scm_prin1 and then passed around to all printing functions as an
 	argument.  A cache `print_state_pool' enables reuse of print
 	states.
	(scm_make_print_state): New function.
	(scm_iprin1): Adaption to print states.
	(scm_iprlist): An initial "hare and tortoise" scan brings down
	time complexity to O (depth * N).  (Better time complexity will be
	achieved when the printing code is completely rewritten.)

	* print.c, print.h: Closures now print like #<procedure foo (x)>.
	People who whish to see the source can do `(print-enable 'source)'.
	Removed #ifdef DEBUG_EXTENSIONS.