Improved printing for tagless structs and unions

[yallop]

Closes #249.

Here are the examples from #249:

let struc = Ctypes.structure ""
let r = field struc "r" float
let g = field struc "g" float
let b = field struc "b" float
let a = field struc "a" float
let () = seal struc

let uni = union ""
let floats = field uni "rgba" (array 4 float)
let struc' = field uni "" struc
let () = seal uni

let struc'' = structure "NVGcolor"
let _ = field struc'' "" uni
let () = seal struc''

And here's how those types are printed before and after this change:

Unions with tagless struct fields

Before

# uni;;
- : '_a Static.union typ = union  { float rgba[4]; struct  ;  }

After

# uni;;
- : '_a Ctypes_static.union typ =
union { float rgba[4]; struct { float r; float g; float b; float a;  } ;  }

Structs with tagless union fields

Before

# struc'';;
- : '_a Static.structure typ = struct NVGcolor { union  ;  }

After

# struc'';;
- : '_a Ctypes_static.structure typ =
struct NVGcolor {
  union { float rgba[4]; struct { float r; float g; float b; float a;  } ; 
  } ; 
}