0.8.17.24:

0.8.17.24:
	Reinstate fixnum arithmetic when possible by defining modular
	  arithmetic mechanisms for (UNSIGNED-BYTE 29)
	... this feels like a big, ugly hack, since the compiler is
	      (presumably) smart enough to do this when modular
	      arithmetic was not present;
	... move some EVAL-WHEN macros into a MACROLET while we're at it;
	... builds and passes tests on x86/Linux; will probably build
	      properly on other platforms, but will fail tests in
	      (at least) tests/arith.pure.  These test failures seem
	      harmless enough and will be fixed in another revision or
	      two.