Rollup merge of #149610 - quaternic:gather-scatter-bits-bench, r=Mark-Simulacrum

Implement benchmarks for uN::{gather,scatter}_bits

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: rust-lang/rust#149069
Accepted ACP: rust-lang/libs-team#695 (comment)

For each method, there are three benchmarks, which differ in that the mask (second) argument is one of:
 - constant at compile time
 - runtime value but invariant for the measured loop
 - different for each call

Sample output
```text
    num::int_bits::u32::constant::gather_bits      555.82ns/iter   +/- 22.41
    num::int_bits::u32::constant::scatter_bits     545.45ns/iter  +/- 124.26
    num::int_bits::u32::invariant::gather_bits    8178.86ns/iter  +/- 217.37
    num::int_bits::u32::invariant::scatter_bits   7135.95ns/iter  +/- 214.51
    num::int_bits::u32::variable::gather_bits    10539.29ns/iter  +/- 198.90
    num::int_bits::u32::variable::scatter_bits    9671.26ns/iter  +/- 254.88
```
(and similarly for the other `uN` types)

Author: matthiaskrgr