Implicit struct initialization: handle fields at any depth?

[RikkiGibson]

The current implementation of implicit struct initialization in #59788 only initializes fields directly contained in 'this', but it's possible that it's more optimal to initialize nested fields in some scenarios.

SharpLab

using System;

public struct Pair
{
    public int X, Y;
}

public struct S {
    public Pair Pair;
    
    public S()
    {
        // Here we insert `Pair = default;` but we could perhaps insert `Pair.Y = default;`
        Pair.X = 42;
    }
}

[jcouv]

From chat with Rikki, triaged this into Compiler.Next milestone. Let's do a quick test/benchmark. Depending on the evaluation, we can address or close.