[2019-06] [runtime] Make cross-appdomain wrappers throw NIE for methods with ref struct args

[monojenkins]

If we have a ref struct and a method that takes it as an argument:

[Serializable]
public ref struct R {
  public int i;
}

public interface I {
  public void M1 (R r, SomeClass o);
}

public class M : MarshalByRefObject, I {
  public void M1 (R r, SomeClass o) { ... }
}

If we create an instance of M in another domain and try to call it:

  I obj = (I)otherDomain.CreateInstanceAndUnwrap (typeof(M).Assembly.FullName,
  nameof(M));
  R r;
  r.i = 42;
  SomeClass o = ...;
  obj.M1 (r, o);

Mono will need to create a wrapper that can invoke M.M1 in the other domain. The way the wrapper works in mono_marshal_get_xappdomain_invoke is by creating an array of all the arguments that need to be serialized (see mono_get_xdomain_marshal_type) and then serializing the whole array (by calling out to System.Runtime.Remoting.RemotingServices.SerializeCallData). In the example it would make a two element array and try to serialize r and o into it. For valuetypes it normally does this by boxing the argument. However since R is a ref-struct, boxing it is not allowed.

This works on .NET Framework.

However for Mono we would need to serilize R without boxing it, which seems challenging with our current setup.

So for now we generate a wrapper that just throws a NotImplementedException if it is ever called.

This fixes #14809 (in that marshalling a TextWriter across domains works again) but in an unsatisfactory way (because you can't call any of the ReadOnlySpan<char> overloads on the transparent proxy object).

---

Also mark MethodInfo and MethodBase with [Serializable] again so that unit testing with NUnit.ApplicationDomain works again (they need to serialize method info for stack traces, I think)

Backport of #14863.

/cc @lambdageek

[akoeplinger]

@monojenkins commit apidiff