documented swift closures

[stephen-hawley]

Added docs that spell out how Swift closures are represented in memory and the calling conventions.

[rolfbjarne]

Maybe non-escaping closures can be modeled using a ref struct? Ref structs "are allocated on the stack and can't escape to the managed heap."

Calling the closure might end up a little bit weird in C# though, since I think you'd have to call a method on the struct instead of using delegate syntax.

ref struct MyNonEscapingClosure {
    public void Invoke (...)
    {
		// call the closure here
    }
}