Handle Ecto structures status through FSM

Provides macros for defining FSM

Defines FSM with transition/2 and bypass/2 macros.

Caller module is added the following functions:

• fsm() :: Ecto.FSM.specs()
• docs() :: Ecto.FSM.docs()

Transition functions must return Ecto.FSM.transition_ret.

Examples:

iex> defmodule Elixir.Door do
...>   use Ecto.FSM
...> 
...>   @doc "Close to open"
...>   @to [:opened]
...>   transition closed({:open, _}, s) do
...>     {:next_state, :opened, s}
...>   end
...> 
...>   @doc "Close to close"
...>   transition closed({:close, _}, s) do
...>     {:next_state, :closed, s}
...>   end
...> 
...>   transition closed({:else, _}, s) do
...>     {:next_state, :closed, s}
...>   end
...> 
...>   @doc "Open to open"
...>   transition opened({:open, _}, s) do
...>     {:next_state, :opened, s}
...>   end
...> 
...>   @doc "Open to close"
...>   @to [:closed]
...>   transition opened({:close, _}, s) do
...>     {:next_state, :closed, s}
...>   end
...> 
...>   transition opened({:else, _}, s) do
...>     {:next_state, :opened, s}
...>   end
...>
...>   @doc "Force the door"
...>   bypass force(_, s) do
...>     {:next_state, :destroyed, s}
...>   end
...> end
...> Door.fsm()
%{
  {:closed, :close} => {Door, [:closed]}, {:closed, :else} => {Door, [:closed]},
  {:closed, :open} => {Door, [:opened]}, {:opened, :close} => {Door, [:closed]},
  {:opened, :else} => {Door, [:opened]}, {:opened, :open} => {Door, [:opened]}
}
...> Door.docs()
%{
  {:transition_doc, :closed, :close} => "Close to close",
  {:transition_doc, :closed, :else} => nil,
  {:transition_doc, :closed, :open} => "Close to open",
  {:transition_doc, :opened, :close} => "Open to close",
  {:transition_doc, :opened, :else} => nil,
  {:transition_doc, :opened, :open} => "Open to open",
  {:event_doc, :force} => "Force the door"
}