Implement fast float parsing and to_s #9162

There's some interesting patches from @mensfeld for improving the performance of string-to-float and float-to-string in CRuby:

• Add Eisel-Lemire algorithm for faster String#to_f ruby/ruby#15655
• Optimize Float#to_s with Ryu algorithm to achieve 2-4x performance ruby/ruby#15627

I have started to implement fast string-to-float via the FastDoubleParser Java library in #9150. We should try to finish that up and also implement fast float-to-string.