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.

Related to:
aldanor#32

[Alexhuszagh]

Is there interest in this? It's definitely doable but would require a bit of a face lift which I'm not sure if any dependents are interested in. Especially making any major code changes to this.

If there's any interest, particularly by the larger users of fast-float2, I'd be more than happy to contribute these changes.