datum labels #N= now star

[Per Bothner]

FWI I changed the printing of datum labels (i.e. #N=DATUM)
to start at 0 rather than 1.  The former seems more common
among Scheme implementations, and matches the example in R7RS.

BTW the write procedure now prints datum labels if a cycle
is detected, but not for other shared structure.  Use write-shared
for the latter, or write-simple to save time and space by
avoiding cycle detection. This matches the R7RS specification.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/