Issue67: Allow "this" to be passed by reference (to methods that use the "in" modifier on the parameter)

[mike-barnett]

Fix Writer.cs so that "this" can have its address taken just as other parameters already could have. A small repro that illustrates this is:

public class C { public void Foo(int x) { Bar(this, x); } public void Bar(in C c, in int y){} }

Both arguments are passed by reference.