Bug fixes:

Bug fixes:

  . "1/"  returns #F instead of signalling divide by zero.

  . "1.2345e-306" and "1e-400" with FLONUM-PARSER-FAST? true no longer
     signal arithmetic errors.

  . STRING->NUMBER and SUBSTRING->NUMBER now check their arguments

Other changes: FINISH-REAL now exploits the fact that some integers
have exact floating point representations and so flonum arithmetic can
be used to compute the correct result.  Note that to maintain
correctness, i*10^-e must be calculated as i/10^e.  The results are a
modest improvement on free-formatted random flonums (typically 16
digit) in the range 1e-7 to 1e35:

Times for 10000 flonums generated by (random range)

range	1e-100	1e-10	1e-5	1.	1e5	1e10	1e35	1e40
old	10180	8200	8210	7490	5610	3970	3990	4030
new	10280	8350	4690	4390	3930	3230	3180	4170

With FLONUM-PARSER-FAST? true, using the division trick gives better
error rates over a larger range.  Number of errors in 10000 random
flonums

range	1e-20	1e-10	1e-6	1.	1e10	1e20	1e35	1e40
old	3573	2400	3309	2798	3025	 685	 722	2730
new	2594	2778	1907	1010	 761	 685	 772	2730