[Enhancement] Support constructor invocation on structs

[thargy]

For example:

    public struct SimpleConstructedStruct
    {
        public readonly float OutFloat;

        public SimpleConstructedStruct(float outFloat)
        {
            OutFloat = outFloat;
        }
    }

...
    SimpleConstructedStruct s = new SimpleConstructedStruct(1.0f);

Should become:

    struct SimpleConstructedStruct
    {
        float OutFloat;
    };

    SimpleConstructedStruct_0_ctor(float outFloat)
    {
        SimpleConsructedStruct this;
        this.OutFloat = outFloat;
        return this;
    }

...
    SimpleConstructedStruct s = SimpleConstructedStruct_0_ctor(1.0);

[mellinoe]

This would be fantastic -- it would also be fantastic if "readonly struct" works. Shaders shouldn't care about it, so we'd just need to make sure that it's ignored in whatever way is needed.