e.g. it seems to me that
```python
class Foo(Elaboratable):
    def __init__(self):
        self.i_foo = Signal(4)
        self.o_bar = Signal(12)
```

now seems more like

```python
FooIn = Signature({
    "foo": Out(4)
})

FooOut = Signature({
    "bar": Out(12)
})

class Foo(Component):
    src: In(FooIn)
    out: Out(FooOut)
```