I think I don't quite understand struct, id doesn't seem possible to create a struct where the elements shape can be controlled like with the StructLayout:

```python
class CFixStruct(data.Struct):
    def __init__(self, shape):
        super().__init__({
            "real": shape,
            "imag": shape,
        })
```

yields:

```python
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[158], line 1
----> 1 CFixStruct(fixed.SQ(8, 8))

File ~/Workspace/research/amaranth/amaranth/lib/data.py:1073, in _AggregateMeta.__call__(cls, target)
   1071 def __call__(cls, target):
   1072     # This method exists to pass the override check done by ShapeCastable.
-> 1073     return super().__call__(cls, target)

File ~/Workspace/research/amaranth/amaranth/hdl/_ast.py:386, in ShapeCastable.__call__(self, *args, **kwargs)
    358 def __call__(self, *args, **kwargs):
    359     """__call__(obj)
    360 
    361     Lift a :ref:`value-like object <lang-valuelike>` to a higher-level representation.
   (...)
    384     A :ref:`value-like object <lang-valuelike>`.
    385     """
--> 386     return super().__call__(*args, **kwargs)

TypeError: CFixStruct.__init__() takes 2 positional arguments but 3 were given
```