Add union support for windows-rdl

[riverar]

Building on #3861, this update adds end-to-end support for unions.

An example in .rdl:

mod Test {
    union Name {
        a1: u8,
        a2: u16,
        a3: f32,
    }
}

The reader will encode this interface into a .winmd file as such:

.class public explicit ansi sealed Test.Name
  extends [mscorlib]System.ValueType
{
  .field [0] public uint8 a1
  .field [0] public uint16 a2
  .field [0] public float32 a3
}

[kennykerr]

That's excellent - thank you!