Refactor v2: faster pop/reserve/retain_mut, modern Criterion benches, Miri-friendly tests + fmt in CI

[fereidani]

Hey,
I did some improvements for the library.

What’s in here:

• Switched the benchmarks over to Criterion and used black_box for correct more accurate benchmark and added retain_mut benches so the Vector trait is fully covered now.
• Made the tests less time consuming when running under Miri.
• Reworked retain_mut, reserve/reserve_exact, and pop. They’re cleaner now and faster.
• Added a fmt check to CI to keep the library formatted.

here is the benchmark result for retain function:

bench_retain_mut_half   time:   [76.825 ns 76.946 ns 77.077 ns]
                        change: [−15.591% −15.307% −15.010%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  4 (4.00%) high mild
  5 (5.00%) high severe

bench_retain_mut_half_small
                        time:   [17.323 ns 17.376 ns 17.434 ns]
                        change: [−5.3509% −4.9664% −4.5552%] (p = 0.00 < 0.05)
                        Performance has improved.

bench_retain_mut_all    time:   [26.311 ns 26.383 ns 26.457 ns]
                        change: [−2.3774% −1.9115% −1.4710%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) low mild
  2 (2.00%) high mild
  1 (1.00%) high severe

bench_retain_mut_all_small
                        time:   [17.984 ns 18.211 ns 18.453 ns]
                        change: [+0.7628% +2.1217% +3.4929%] (p = 0.00 < 0.05)
                        Change within noise threshold.

bench_retain_mut_none   time:   [25.847 ns 25.917 ns 25.994 ns]
                        change: [−1.2565% −0.3335% +0.5576%] (p = 0.48 > 0.05)
                        No change in performance detected.
Found 11 outliers among 100 measurements (11.00%)
  11 (11.00%) high severe

bench_retain_mut_none_small
                        time:   [12.992 ns 13.038 ns 13.088 ns]
                        change: [−0.0966% +0.7096% +1.4743%] (p = 0.08 > 0.05)
                        No change in performance detected.
Found 10 outliers among 100 measurements (10.00%)
  5 (5.00%) high mild
  5 (5.00%) high severe

Thanks you for maintaining this library.

[fereidani]

Forgot to add that rework of reserve is based on std implementation of reserve.