[plt-scheme] Cost of string creation? / Euler 8 code review

5000 operations isn't really much, even if each operation is somewhat
"expensive".  I think it would be pretty hard to come up with a
solution to this problem that was noticeably slow.  But my intuition
is that your program would be faster if you converted the whole thing
to a list of digit numbers up front, and then wrote the solution as a
recursive list function.

--Mark