[RFC] Add grisu3 strtod algorithm

Hi Internals, I want to propose the inclusion of Florian Loitschs (called grisu3) strtod algorithm. Essentially Florian found a new algorithm for string to double conversion and vice versa. According to micro benchmarks in PHP and experiences with the library in the haskell community, this can speed up heavy conversions like json_decode up to 20-30%. It is a backward compatible change as we will not change the underlying zend_strtod api. The proposed change is enabled with a flag and will be disabled by default. As no underlying API is affected and because it's disabled by default, I propose this change to be included in PHP 5.4. I'll finish the patch before alpha3. The RFC an be found here: https://wiki.php.net/rfc/grisu3-strtod - David