Optimize Performance for Slow-Path Cases

[Alexhuszagh]

Recently, upon a major rewrite of lexical, noticed that although performance for fast_float and fast-float Rust was by far the most performant of any library, for near-halfway cases, especially those with many digits or large exponents, the performance dwindled rapidly. A detailed description can be found in fastfloat/fast_float#93, and a PR to fix the aforementioned issue can be found in fastfloat/fast_float#96.

As with fast_float, I own all the code required, and am willing to submit it under any license, including public domain. It might take me a while to finish the PR, as I am doing some work with other projects and work, but would be more than happy to submit a PR if there is interest.

[lemire]

+1