Make FormulaEngine and FormulaEngine3Phase composable

- Also make `FormulaEngine` take a `FormulaBuilder` as constructor
  param, so that subscriptions are made directly from the
  FormulaEngine's task,  thereby avoiding async calls when creating
  formulas.

- Also remove all generic types, and use Union types instead.
  Overloads will be added in a subsequent commit, to fix any remaining
  typing issues as well.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms