* libguile/eval.c: Separated some definitions relevant for execution

	* libguile/eval.c: Separated some definitions relevant for execution
	from the memoization part of the file.

	(copy_tree): New static function

	(scm_copy_tree): Rewritten to fix two kinds or bugs: First, cyclic
	structures are detected now and will lead to an exception instead
	of forcing guile to run in an endless loop, using up all the
	system's memory.  Second, arrays in the cdr of an improper list
	are now copied.  See the new test cases in eval.test.

	* test-suite/tests/eval.test: Added tests which reflect the recent
	fixes to copy-tree.