Tracking Issue for signed bigint helper methods

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Feature gate: #![feature(signed_bigint_helpers)]

This issue continues discussion #85532 with the following methods on integers:

• uN::carrying_mul_add_signed
• iN::overflowing_add_carry
• iN::overflowing_sub_borrow
• iN::carrying_mul
• iN::carrying_mul_add
• iN::carrying_mul_add_unsigned

impl uN {
    /// `add += self * rhs + carry`, covers full `i2N` range exactly
    fn carrying_mul_add_signed(self, rhs: iN, carry: Self, add: iN) -> (Self, iN);
}

impl iN {
    /// Returns `self + rhs + carry` and (signed) overflow flag;
    /// should be used for most significant limb only; shouldn't be chained
    fn overflowing_add_carry(self, rhs: Self, carry: bool) -> (Self, bool);
    
    /// Returns `self - rhs - borrow` and (signed) overflow flag;
    /// should be used for most significant limb only; shouldn't be chained
    fn overflowing_sub_borrow(self, rhs: Self, borrow: bool) -> (Self, bool);
    
    /// `self * rhs + carry`
    fn carrying_mul(self, rhs: Self, carry: Self) -> (uN, Self);
    
    /// `add += self * rhs + carry`, fits `i2N` range, but doesn't cover it fully
    fn carrying_mul_add(self, rhs: Self, carry: Self, add: Self) -> (uN, Self);
    
    /// `add += self * rhs + carry`, covers full `i2N` range exactly
    fn carrying_mul_add_unsigned(self, rhs: uN, carry: Self, add: uN) -> (uN, Self);
}

Start by reading issue #85532 and the context for the signed_bigint_helpers feature gate. Use the listed method signatures and semantics to identify unresolved API and design decisions. Done means the scope is agreed and the signed bigint helpers are implemented with appropriate validation.