num: Implement uint_gather_scatter_bits feature for unsigned integers

[okaneco]

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

Implement gather_bits, scatter_bits functions on unsigned integers
Add tests to coretests

This implementation is a small improvement over the plain naive form (see the solution sketch).
We only check the set bits in the mask instead of iterating over every bit.

[rustbot]

r? @Mark-Simulacrum

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