Enhance HAL_Shutdown to actually reset HAL

[virtuald]

Is your feature request related to a problem? Please describe.
RobotPy wants integration tests that can be ran in a single process with multiple robot class resets.

Describe the solution you'd like
Add all simulation reset knowhow to HAL_Shutdown. This stuff will need to re-arm HAL_Shutdown so that it can be called after each test.

• HAL_Shutdown should be moved out of extension.h

Additional context
I'm going to implement an initial version in RobotPy's HAL, and we'll see what issues we run into. Thing I know will need to add themselves to HAL_Shutdown (will add to this list over time):

• Command scheduler should reset itself
• SendableRegistry should reset itself
• NetworkTables would be nice, but I think it'll have to stay separate

[Starlight220]

@virtuald what's the current status of this? I remember seeing multiple shutdown-related PRs. What still needs to be done?