Add STL's general formatting for ryu_printf

[Artoria2e5]

This is a very early draft for putting %g in ryu so that printf() writers will find it easier to use it. Current TODOs include:

• tests (I haven't even tried building it! There is a near 100% chance something fails.)
• java code for table generation

Credits go to @StephanTLavavej for his C++ code.

[ulfjack]

Cool!

[maelh]

I was trying to get more info on your commit, but the link https://reviews.llvm.org/file/data/kb2knv5sxt2jhdfj25ah/PHID-FILE-6wgegsparpeqk3ror2gv/libcxx_include_charconv is broken.

[Artoria2e5]

Uh, fine. Try https://reviews.llvm.org/D70631. Thanks for the reminder by the way -- my life is fucked as usual, but maybe I can actually get it rolling sometime.