[API Proposal]: `BigMul` for `nuint` and `nint`

[MineCake147E]

Background and motivation

.NET 9 lacks BigMul API for nuint for some reason.
BigMul is useful for implementing something like multiplicative hashing, decoding factoradic with fixed max input value, Lemire's algorithm for generating bounded random integers efficiently, and fast fraction multiplications (necessary for something like MemoryMarshal.Cast<Vector4, Vector3>()).

API Proposal

namespace System
{
    public readonly struct UIntPtr
    {
        public static nuint BigMul(nuint a, nuint b, out nuint low);
    }

    public readonly struct IntPtr
    {
        public static nint BigMul(nint a, nint b, out nint low);
    }
}

API Usage

var sb = new StringBuilder("0.");
var low = ~(nuint)0;
while (low > 0)
{
    sb.Append($"{nuint.BigMul(10, low, out low)}");
}
Console.Out.WriteLine(sb);

Alternative Designs

Risks

None

[tannergooding]

You'd need to update the proposal to expose it on nint/nuint instead. It should always be done in pairs and we're no longer versioning System.Math or System.MathF.

[huoyaoyuan]

Given there is already (TSelf, TSelf) DivRem(TSelf, TSelf) in generic math, would it make sense to also expose (TSelf, TSelf) BigMul(TSelf, TSelf) in generic math?

Update: there is already Int128 BigMul(Int64, Int64), which conflicts with the overload. However, there's no definition of "2*nint" type.