Simplify some literal-value negations with u128::wrapping_neg

[Zalathar]

• Calling overflowing_neg and then discarding the overflow bit is the same as just calling wrapping_neg.
• Casting to u128 to i128, negating, and then casting back produces the same result as just performing an unsigned wrapping_neg.

There should be no actual change to compiler behaviour.

[rustbot]

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