Re-vendor and re-patch grisu files

[jennybc]

I'd like to start this over from the beginning and record our changes as patch files that we apply. Motivation is 50/50 really wanting to have such a structured provenance (so it's totally clear what modifications we've made to the code), but also wanting to start codifying this sort of repetitive task (like what we do for data-raw/, but for vendored code r-lib/usethis#1698).

I think starting over would begin with source files from here, e.g.:

https://github.com/juj/MathGeoLib/blob/master/src/Math/grisu3.c

(which now contains a patch from us, deleting a rogue nonprinting character).

Helpful commands / process: https://www.thegeekstuff.com/2014/12/patch-command-examples/