SendableRegistry Add/AddLW/AddChild are broken

They are trying to use keep alives to keep the pointers alive but it's not the right solution.

Probably need to do the same thing we do for SmartDashboard with a separate hash map to hold references.

```python
import wpilib
import wpilib.drive
import wpiutil

leftLeader = wpilib.PWMSparkMax(1)
rightLeader = wpilib.PWMSparkMax(2)
drive = wpilib.drive.DifferentialDrive(
leftLeader,
rightLeader,
)

wpiutil.SendableRegistry.addChild(drive, leftLeader)
```

```sh
$ python ../../t.py
Traceback (most recent call last):
File "../../t.py", in
wpiutil.SendableRegistry.addChild(drive, leftLeader)
RuntimeError: Could not activate keep_alive!
```