The existing implementation was not very good:
 - It printed all very small values as "0.000000..."
 - It did not distinguish positive and negative zero
 - In some cases it printed extra garbage digits for single-precision values
   (e.g. 3.9e+30 was printed as 3.90000013+30)

Tests have been updated to check that we now match printf("%.6g") for
1,000,000 randomly chosen values, except for rounding of the 6th digit.