Refactor Anima#initialize to use Method#to_proc
Refactor Anima::Attribute#load to use the key from Hash#fetch
Remove #attributes_hash from anima descendant classes

* The #to_h on the instance provides everything that is needed, and
  it is no longer necessary to pollute the class namespace with a
  method that acts on instance state.
Refactor define_anima_method to be consistent with other methods
Fix mutation in Anima::Error