remove unsafe from float.rs at very small performance cost

[Protowalker]

OLD:

min/median/max

ns/float: 22.29/23.64/28.55
Mfloat/s: 44.86/42.31/35.03
MB/s: 781.57/737.14/610.28

NEW:
ns/float: 22.9/24.94/30.69
Mfloat/s: 43.67/40.10/32.59
MB/s: 760/87/698.71/567.77

I'm going to continue working on this to try and improve it as much as possible.

[aldanor]

Hi, thanks for attempting this :)

3% may not seem like a lot but it still is a fairly tangible difference if it's consistent (3% here, 3% there add up to quite a lot in the end). I would suggest testing it mostly on canada.txt and uniform and, as extreme case, on mesh.txt.

[shumpohl]

I totally agree that a consistent 3% advantage in such a low-level libraray justifies the usage of unsafe if unavoidable. But I think pow10_fast_path itself should be unsafe with an additional comment what the invariants are (exponent < 10) that the caller needs to uphold and a comment on unsafe block at the callside that this unsafe block is fine because you checked it beforehand.

Nice work, btw :)

[Protowalker]

Can I come clean about something? I had only tested my other theory about a speed increase and not this one -- then I wrote this one, and then I made the PR, and then before I hit submit I realized I hadn't run benchmarks, so I ran some and when I realized it was slower I just changed the title from "improving performance" to "with low performance cost" 😬

I'm gonna do another implementation with the idea that I have tested but requires a larger rewrite than a couple lines, then I'll open this up again. Cheers!