0.7.1.23:

0.7.1.23:
	catching stack overflow, part I...
	...defined placeholder %DETECT-STACK-EXHAUSTION
	...arranged for it to be called on entry to lambdas when
		(OR (> SAFETY (MAX SPEED SPACE)) (= SAFETY 3))
	...downgraded SAFETY to 2 (= SPEED) in SBCL's own code, since
		SBCL's own code seems to be at little risk of infinite
		recursion