Release 0.10.0 with refined float to string conversion

[thiemowmde]

I played around with the float-to-string conversion, tried a lot of different approaches I already tried before, and found a new one:

• I looked into PHP's "precision" setting. It defaults to 14. I found this problematic for two reasons:
  1. After #115 this code started to depend on local configuration.
  2. I found 14 not enough. ECMAScript for example outputs about 17 significant digits.
• I tried json_encode, but found it uses the same "precision" setting.
• I found a "serialize_precision" setting that defaults to 17. This fits much better to what this code is meant to do: The string is meant to be a serialization, not to be looked at by a human. With 17 places the output might end with 1 or 2 partially messed up digits. For example, you might end with "0.99999999999999989" instead of the expected "0.99999999999999999". This is because of the IEEE conversion. What's relevant: With 17 places the string is guaranteed to become the same IEEE floating point number when converted back.

This patch fixes two of the listed issues, and (I believe) avoids all others:

• The default of 17 significant digits is used and enforced.
• Because it now uses PHP's serialize(), the locale does not matter any more.

Bug: T155910

[mariushoch]

Looking at the PHP sources (https://github.com/php/php-src/blob/414aaa207cc59c279d3fd5ed83f6b92890e57023/Zend/zend_operators.c#L560), I think you could also use sprintf("%.17G", $number); here (although it is to note that this is probably not using the same code path as serialize does, which uses gcvt internally).

Although sprintf can use a localized decimal point, which serialize can't (hard coded to .).

[thiemowmde]

I heavily updated the patch to use sprintf with %e now. That was new to me (and is not well documented, unfortunately). It indeed allows to format floats with a fixed number of significant digits, which is what we need here.