Make Signed::abs match its documented behavior for ::MIN

[tautschnig]

The Signed trait documentation states: "For signed integers, ::MIN will be returned if the number is ::MIN." The primitive implementations use unary negation, which panics with "attempt to negate with overflow" when overflow checks are enabled (debug builds):

let r = num_traits::abs(i32::MIN); // debug: panic; release: returns i32::MIN as documented

This PR switches the implementation to wrapping_abs, which implements the documented semantics in both build profiles, and adds a regression test (which runs with overflow checks enabled by default, so it would panic without the fix).

It also documents the overflow behavior of abs_sub for signed integers (e.g. i32::MAX.abs_sub(&-1) is not representable), which panics with overflow checks enabled and wraps otherwise; changing that behavior (e.g. saturating) seemed like a semantic decision better left to the maintainers, so this PR only documents it.

Found by running Kani's autoharness (model-checking/kani#3832) over the crate: the generated harness for abs::<i32> reported the negate-overflow panic reachable; reproduced with plain cargo (no Kani involved) before filing.

[Copilot]

Pull request overview

Updates Signed::abs for primitive signed integers to match the trait’s documented ::MIN behavior in debug (overflow-checking) builds, and adds a regression test to prevent reintroducing the overflow panic.

Changes:

• Switch primitive signed integer Signed::abs implementation from negation to wrapping_abs to avoid ::MIN negate overflow panics.
• Add a regression test ensuring abs(::MIN) == ::MIN across signed integer primitives and the sign::abs helper.
• Document the overflow behavior of Signed::abs_sub for signed integers when the difference is not representable.

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