Simplify aggregate root requirements

[slashdotdash]

Remove dependency on eventsourced library and allow Commanded to use any Elixir module that follows these conventions.

Command functions accept the aggregate root state and the command. They return the events that have resulted. This may be zero (nil or []), one, or more.

defmodule Account do
  defstruct [account_number: nil, balance: 0]

  defmodule Deposit, do: defstruct [:account_number, :amount]
  defmodule MoneyDeposited, do: defstruct [:account_number, :amount]
  
  # command function
  def execute(%Account{}, %Deposit{amount: amount}) when amount > 0 do
    %MoneyDeposited{amount: amount}
  end

  # state mutator
  def apply(%Account{balance: balance} = account, %MoneyDeposited{amount: amount}) do
    %Account{account | balance: balance + amount}
  end
end

Tracking raised events, aggregate version, and calling the apply state mutator will be handled by theCommanded.Aggregates.Aggregate process.

The expected execute and apply functions could be specified as a behaviour (e.g. Commanded.Aggregates.AggregateRoot).