[plt-scheme] Changes to SRFI 19

Hello,

Francisco Solsona and I have made some changes to the SRFI 19 
implementation in the cvs repository.  Since SRFI 19 defines names that 
conflict with primitives in the mzscheme language, two modules are 
provided:  srfi/19.ss provides the names as defined in the SRFI 
document, srfi/19/time.ss provides srfi: prefixed identifiers where 
clashes with mzscheme occur (eg. srfi:make-date, srfi:date?, etc). 
19.ss is meant to be used from the top-level to write portable programs, 
whereas time.ss is convenient to use when writing PLT modules.  This 
change makes SRFI 19 consistent with the way SRFI 1 is packaged.

Any comments are of course welcome.

Thanks,
David