Adding combine algebra on the Decider, View and Saga

[idugalic]

Decider

/// Combine two deciders into one bigger decider
/// Creates a new instance of a Decider by combining two deciders of type `C1`, `S1`, `E1` and `C2`, `S2`, `E2` into a new decider of type `Sum<C, C2>`, `(S, S2)`, `Sum<E, E2>`
pub fn combine<'a, C1, S1: Clone, E1, C2, S2: Clone, E2>(
    decider1: Decider<'a, C1, S1, E1>,
    decider2: Decider<'a, C2, S2, E2>,
) -> Decider<'a, Sum<C1, C2>, (S1, S2), Sum<E1, E2>>

View

/// Combine two views into one.
/// Creates a new instance of a View by combining two views of type `S1`, `E1` and `S2`, `E2` into a new view of type `(S1, S2)`, `Sum<E1, E2>`
pub fn combine<'a, S1: Clone, E1, S2: Clone, E2>(
    view1: View<'a, S1, E1>,
    view2: View<'a, S2, E2>,
) -> View<'a, (S1, S2), Sum<E1, E2>>

Saga

/// Combine two sagas into one.
/// Creates a new instance of a Saga by combining two sagas of type `AR1`, `A1` and `AR2`, `A2` into a new saga of type `Sum<AR1, AR2>`, `Sum<A1, A2>`
pub fn combine<'a, AR1, A1, AR2, A2>(
    saga1: Saga<'a, AR1, A1>,
    saga2: Saga<'a, AR2, A2>,
) -> Saga<'a, Sum<AR1, AR2>, Sum<A1, A2>>

Combined decider/view/saga would allow you to create single aggregate/materialized view/saga manager to handle all the commands/events of your system. Could you check the tests?