[generator] Use DIM to support static interface methods.

[jpobst]

Using the new DIM support we can now also support static interface methods/properties.

Example:

public interface StaticMethodsInterface
{
    static int foo () { return 10; }
    static int getValue () { return 3; }
    static void setValue (int value) { }
}

Can now be invoked:

[Test]
public void TestStaticMethods ()
{
    Assert.AreEqual (10, IStaticMethodsInterface.Foo ());
			
    Assert.AreEqual (3, IStaticMethodsInterface.Value);
    Assert.DoesNotThrow (() => IStaticMethodsInterface.Value = 5);
}

[jonpryor]

Why are we emitting an empty MyInterfaceConsts type? If the interface had some constants, that would be one thing, but it doesn't...

[jpobst]

The MyInterfaceConsts type is always generated if we generate the MyInterface class. It does not attempt to detect why we are writing a MyInterface class.

Logic for determining if we are writing a MyInterface class:

interface.Fields.Any () || interface.Methods.Where (m => m.IsStatic)

[jonpryor]

Should we alter that logic? Particularly considering that the generated type is [Obsolete] (lol?), and there's no reason for the type to exist in a C#8 default-interface-members world order?

[jpobst]

This is technically a break if an existing library enables DIM, so we discussed making these [Obsolete(error:true)] so maybe eventually we could remove them. (For a separate PR.)