Grisu2

This is a C++11 implementation of the Grisu2 algorithm for converting floating-point numbers to decimal strings.

The Grisu2 algorithm is by Florian Loitsch, based on the work of Robert G. Burger and R. Kent Dybvig:

• https://dl.acm.org/doi/10.1145/1806596.1806623 [1] Loitsch, "Printing Floating-Point Numbers Quickly and Accurately with Integers", Proceedings of the ACM SIGPLAN 2010 Conference on Programming Language Design and Implementation, PLDI 2010
• https://dl.acm.org/doi/10.1145/231379.231397 [2] Burger, Dybvig, "Printing Floating-Point Numbers Quickly and Accurately", Proceedings of the ACM SIGPLAN 1996 Conference on Programming Language Design and Implementation, PLDI 1996

The original C implementation is by Florian Loitsch:

• https://drive.google.com/file/d/0BwvYOx00EwKmejFIMjRORTFLcTA/view?resourcekey=0-1Lg8tXTC_JAODUcFpMcaTA

The implementation simplified and adapted to JSON and C++11 by Daniel Lemire as part of simdjson:

• https://github.com/simdjson/simdjson/blob/master/src/to_chars.cpp

The grisu2.h file is the same as to_chars.cpp but with godot.patch applied to it, with the following changes:

• Simplify namespaces to just be one grisu2 namespace.
• Rename functions to ensure their names are unique.
• Make to_chars handle both float and double types instead of just double.
• Remove the trailing .0 logic to match Godot's existing String::num_scientific behavior.