Write u128 using only two divisions

[dtolnay]

@henninglive this one is a little faster than your implementation from #10. Basically it decomposes N into (N / 10^38 % 10) and (N / 10^19 % 10^19) and (N % 10^19).

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Unoptimized

test bench_fmt::bench_u128_max  ... bench:       2,532 ns/iter (+/- 219)

PR #10

test bench_itoa::bench_u128_max ... bench:       1,171 ns/iter (+/- 30)

This one

test bench_itoa::bench_u128_max ... bench:         198 ns/iter (+/- 8)

[henninglive]

Wow, that’s fast. This is probably worth submitting as patch to std.

[henninglive]

We sould alos look into what happens with i64/u64 on x86, can we speed up too.

[henninglive]

The compiler-builtins copyright notice probably doesn’t apply anymore, you can probably remove it.

[varkor]

@dtolnay: are you planning to submit this upstream?

[dtolnay]

@varkor I am not planning to do it but it is being tracked in rust-lang/rust#44583.