rename uN::{gather,scatter}_bits to uN::{extract,deposit}_bits

[quaternic]

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: #149069

Rename the methods as requested in #149069 (comment)

• gather_bits -> extract_bits
• scatter_bits -> deposit_bits

[rustbot]

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[quaternic]

The feature gate name refers to the previous method names. Would it make sense to also rename that?

[scottmcm]

I don't think it's worth making people rename their feature gates. We can rename it during stabilization if we want -- the name doesn't actually matter for anything.

r=me if CI passes