0.7.1.38:

0.7.1.38:
	catching stack overflow, part III...
	...redid *STACK-EXHAUSTION* value as SAP, because although
		FIXNUM is cute and should be implementable efficiently,
		it seems to be awfully annoying to work with
	...made %DETECT-STACK-EXHAUSTION actually check for the
		problem
	...set up machinery to try to handle the problem reasonably
		gracefully
	...added basic regression test
	moved EXPORT of '*SHEBANG-BACKEND-SUBFEATURES* alongside
		EXPORT of '*SHEBANG-FEATURES* so that chill.lisp
		works again