Simplify instantiation of generic-bound union members

[domn1995]

Consider the following union:

[Union]
public partial record Option<T>
{
    public partial record Some(T Value);
    public partial record None;
}

Currently, to instantiate the Some variant, we must write the following:

var some = new Option<int>.Some(42);

Since the generic type parameter is bound to the union, we must define the type parameter every time since it cannot be inferred from the type passed to the union member. It would be nicer to be able to do the following:

// Inferred as `Option<int>.Some` since `42` is an integer.
var someInt = Option.Some(42);
// Inferred as `Option<string>.Some`.
var someStr = Option.Some("foo");

[domn1995]

For backward compatibility, we probably want to expose this through a generated XPrelude static class where X is the union name. Ex:

using OptionPrelude;

var some = Option.Some(42);