Will use as few digits as is necessary to ensure that round-tripping the same number with pstrtod will result in the same vector. This prevents eg. 3.3 formatting as 3.29999995 while still ensuring that two floats that are not equal (other than nan) are guaranteed to have a different string representation.
Uses a hacky pftoa function that can be abandoned as soon as we adopt C++17, which has to_chars that does what we need
Fixes#1671
rdb