Exp/newtype physics

This converts several of the units used in physics calculations to tuple structs with overloaded arithmetic.

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  * `Millis + Millis -> Millis`
  * `Millis - Millis -> Millis`
  * `-Velocity`
  * `Velocity + Velocity -> Velocity`
  * `Velocity - Velocity -> Velocity`
  * `Velocity * Millis -> Game`
  * `Acceleration * Millis -> Velocity`
  * `-Acceleration`