Support Roblox's pivot API

[Anaminus]

It would be wildly convenient if it were possible to use GetPivot and PivotTo to move models and parts around.

Considerations:

• Several properties from several classes are affected.
• Must be descriptor-agnostic. Should be similar to descriptor/AttrConfig API.
• A configuration matching Roblox is provided by default.
• Roblox can add more affected classes/properties in the future. The implementation should be simple to extend.
  • Use descriptor extensions (#65).
• Basically involves implementing Roblox's pivoting engine. Could be daunting.
  • We have a complete CFrame implementation, so it might not be so bad.
• Should have thorough testing!
  • Write tests for Roblox's implementation. Design them so that they can also run in rbxmk.