Use Grisu3 for serializing floats

[dtolnay]

We currently use Grisu2 through dtoa. I see a Grisu3 implementation in rust-strconv. Grisu3 would be more accurate in some cases and is what rustc-serialize uses.

[lifthrasiir]

I will look for providing a dtoa-like interface for rust-strconv's Grisu3 implementation.

[mrhota]

Leaving this here for people who are interested in the subject: https://cseweb.ucsd.edu/~lerner/papers/fp-printing-popl16.pdf

The paper contains description and characterization of Errol algorithm which is more accurate but slower than Grisu3 and faster than the old Dragon4 algorithm.