More effisient formatting of numbers with small decimal mantissas

[plokhotnyuk]

Results for 0.1

Before:

             Average   Stddev
pretty32:      26.083    0.628
pretty64:      52.132    0.331

After:

             Average   Stddev
pretty32:      20.886    0.481
pretty64:      42.843    0.249

Results for 0.12345678

Before:

             Average   Stddev
pretty32:      18.052    0.871
pretty64:      39.528    0.211

After:

             Average   Stddev
pretty32:      18.124    0.726
pretty64:      34.618    0.210

Results for 0.12345678912345678

Before:

             Average   Stddev
pretty32:      18.052    0.604
pretty64:      26.635    0.204

After:

             Average   Stddev
pretty32:      18.123    0.668
pretty64:      27.461    0.151

A big thanks to @andy128k for his help in writing and debugging of these changes.

[dtolnay]

Thanks! I am closing the PR because I don't feel comfortable diverging from the C code at this point. As the readme says, this is a line-by-line port. Being able to port future performance improvements easily from C to Rust is more valuable in expected future performance than this optimization. You could publish this as a fork under a different crate name, or follow up in ulfjack/ryu.