[plt-scheme] srfi-42 dispatch-union not exported.

Hello,

I'm not sure if this is the correct list for the report, but the 
version of srfi-42 provided with PLT Scheme 299.405 (recent SVN 
sources) does not provide the dispatch-union identifier.  It's an easy 
fix: just include dispatch-union in the (provide ...) clause at the 
beginning of comprehensions.ss (i.e. dispatch-union is defined 
appropriately in the file itself; it's just not provided).

Thanks,
Will