zend_double_to_str() converts a double to string in the way that
(string) would (using %.*H using precision).
smart_str_append_double() provides some more fine control over
the precision, and whether a zero fraction should be appeneded
for whole numbers.
A caveat here is that raw calls to zend_gcvt and going through
s*printf has slightly different behavior for the degenarate
precision=0 case. zend_gcvt will add a dummy E+0 in that case,
while s*printf convert this to precision=1 and will not. I'm
going with the s*printf behavior here, which is more common,
but does result in a minor change to the precision.phpt test.