Add: Bit at index in DataInt.Body

Accessing a bit in a memory region is semi-common, but `DataInt.Body` doesn't have a `Shift<T>` bit index as integers do. Well, so I reasoned at some point. `Count` is semantically similar to `Shift<T>`  so I suspect the following subscript would be convenient.

```swift
subscript(unchecked index: Count) -> Bit
```
