Support tuples for OnNewAccount hook in Balances module

[shawntabrizi]

Runtime developers may want to trigger multiple functions when a new account is created, however this is not currently supported for OnNewAccount like it is for other hooks.

We should also look through the other modules and make sure that their hooks also support tuples.

[xlc]

EventHandler pattern is commonly used in C#. Does it make sense to have something similar here to make those callback / event handler easier to use?

Something like

trait EventHandler<T> {
    fn onEvent(args: T);
}

struct SomeEvent {
   foo: u32
}

trait Trait {
    type OnSomeEvent: EventHandler<SomeEvent>;
}

impl<T> EventHandler<T> for () 
impl<T, A: EventHandler<T>, B: EventHandler<T>> EventHandler<T> for (A, B) 
....

impl my_module::Trait for Runtime {
   OnSomeEvent: (ModuleA, ModuleB)
}

The goal is avoid update each individual trigger to make them support multiple handlers.

Also, is there a proper name for this pattern? Trigger? Callback? I will want to call it event but there is already another event in Substrate.