Provides a set of macros to define the state of a Makina model.

  The state of a Makina model is formed by a set of attributes. These attributes are stored inside a
  map with the name of the attributes as keys.

  ## Basic state definition

  A basic state declaration is shown below:

      # 1) Create a module that will store the state
      defmodule Model do
        # 2) Use Makina.State to configure the module and import the necessary macros
        use Makina.State
        # 3) Write the state definition, type declarations are optional but recommended.
        defstate value: 0 :: integer()
      end

  Each attribute in the state definition should contain different names. Multiple state definitions
  are allowed.

  ## Functions

  After macro expansion the module will contain the functions:
  - Each attribute will generate a function `attribute_`name`/0` which contains the initial value of
    that attribute. In this example this is just the function `attribute_value/0`.
  - `initial_state/0` that returns the initial state of the model. In this example the map `%{value:
    0}`.
  - `validate_state/1` that receives a state and throws an error if it doesn't contain any of the
    attributes in the state definition.

  ## Types

  After macro expansion the module will contain the types:
  - `symbolic_state/0` represents the state of the model during test generation.
  - `dynamic_state/0` represents the state of the model during test execution.
  - `state/0` represents the state at any point of the execution.
  - `state_update/0` represents the modifications allowed to the state.

  ## Extending the state

  Makina states can be extended in new models.

      defmodule Extends do
        use Makina.State, extends: Model
        defstate attr_2: 1 :: integer()
      end

  The resulting state will contain the union of the state attributes defined in `Model` and
  `Extends`. Notice that definitions in `Model` can be overriden:

      defmodule Extends2 do
        use Makina.State, extends: Model
        defstate value: super() + 1
      end

  When an attribute is overriden its type definition is not required and the original initial state
  can be accessed using `super/0`.