Improvements to the annotation functionality

[Brian]

I want and I think:

```D

struct A
{
     int x = 1;
     int y = 2;
     int z = 3;
     string a = "a";
     string b = "b";

     this(int x, int y, int z, string a, string b)
     {
         this.x = x;
         thix.y = y;
         this.z = z;
         this.a = a;
         this.b = b;
     }
}

@A(1, 2, 3, "a", "b")
void test1()
{
     // TODO
}


@A(z=9, b="bbb")
void test2()
{
     // TODO
}

```

Member assignments can be specified, seem test2().