Use stabilized `core` bigint helper methods

[tarcieri]

Rust 1.91 is out and stabilized the first round of these: https://blog.rust-lang.org/2025/10/30/Rust-1.91.0/

• u{N}::carrying_add
• u{N}::borrowing_sub
• u{N}::carrying_mul
• u{N}::carrying_mul_add

Note they're not yet usable in const fn, so maybe not quite up to our needs yet.

Edit: per the stabilization PR constifying them is blocked on const traits, so that may be awhile unless the underlying implementation changes