Macro impl_v_field

macro_rules! impl_v_field {
    (
        $tag:ty,
        $field:ident,
        $product:ty,
        $asset:ty,
        $rational:ty,
        $time:ty,
        $addon:ty,
        $value:ty
    ) => { ... };
}

Implements VirtualDynField for a ProductType field.

Maps a discriminant tag to a concrete virtual struct field using SumDynType.

Bounds are enforced via VirtualDynBound provided by ProductProvider.