uucore: num_format: Optimize format_float_shortest

[drinkcat]

Optimizing for the %g printf format. I don't particularly care about the usecase below, but I'm thinking of moving od to the common formatter, where those optimizations would be welcome.

We're still 20% away from coreutils, but this picks some of the low hanging fruits.

cargo build --release -p uu_seq && hyperfine -L seq ./target/release/seq,seq,./seq.main "{seq} -f "%g" 0 1e-9 1e-3 > /dev/null"
    Finished `release` profile [optimized] target(s) in 0.13s
Benchmark 1: ./target/release/seq -f %g 0 1e-9 1e-3 > /dev/null
  Time (mean ± σ):     169.5 ms ±   3.3 ms    [User: 168.0 ms, System: 1.0 ms]
  Range (min … max):   164.4 ms … 174.7 ms    17 runs
 
Benchmark 2: seq -f %g 0 1e-9 1e-3 > /dev/null
  Time (mean ± σ):     138.8 ms ±   4.9 ms    [User: 136.7 ms, System: 1.6 ms]
  Range (min … max):   133.8 ms … 153.5 ms    22 runs
 
Benchmark 3: ./seq.main -f %g 0 1e-9 1e-3 > /dev/null
  Time (mean ± σ):     237.5 ms ±   4.9 ms    [User: 235.4 ms, System: 1.4 ms]
  Range (min … max):   232.6 ms … 249.8 ms    12 runs
 
Summary
  seq -f %g 0 1e-9 1e-3 > /dev/null ran
    1.22 ± 0.05 times faster than ./target/release/seq -f %g 0 1e-9 1e-3 > /dev/null
    1.71 ± 0.07 times faster than ./seq.main -f %g 0 1e-9 1e-3 > /dev/null

---

uucore: num_format: Optimize format_float_shortest

We already know the String length ahead of time, and we can
avoid using format.

Saves about ~30% performance on:

{seq} -f "%g" 0 1e-9 1e-3

uucore: num_format: Reduce calls to with_prec

with_prec is actually really expensive, so it's much better to
just call it once, and handle the rounding corner case manually.

Saves about ~20% performance on:

{seq} -f "%g" 0 1e-9 1e-3

uucore: num_format: Move common scientific formatting code to a function

Both format_float_scientific and format_float_shortest carry the
same code, moving it to a single function will make it possible
to optimize both.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)