dstruct labels should be optional

[evie-calico]

The dstruct macro is useful, but sometimes you want to construct an object without creating any labels.

For example, I use this pattern relatively often in esprit:

struct Foo
  words 1, Pointer
end_struct

dstruct Foo, Foo, .Pointer=.ptr ; Error: Foo_Pointer.ptr doesn't exist
.ptr

I'd prefer to do something like this:

Foo:: dstruct_anon Foo, .Pointer=.ptr
.ptr