Add missing inverse_of's

Without these inverse_of's Rails loads the records even if they have
been eager loaded.

app/models/alchemy/content.rb

@@ -24,7 +24,7 @@ class Content < BaseRecord
     # Concerns
     include Alchemy::Content::Factory
 
-    belongs_to :essence, polymorphic: true, dependent: :destroy
+    belongs_to :essence, polymorphic: true, dependent: :destroy, inverse_of: :content
     belongs_to :element, touch: true, inverse_of: :contents
     has_one :page, through: :element
 

app/models/alchemy/element.rb

@@ -62,7 +62,7 @@ class Element < BaseRecord
 
     # Content positions are scoped by their essence_type, so positions can be the same for different contents.
     # In order to get contents in creation order we also order them by id.
-    has_many :contents, -> { order(:position, :id) }, dependent: :destroy
+    has_many :contents, -> { order(:position, :id) }, dependent: :destroy, inverse_of: :element
 
     has_many :all_nested_elements,
       -> { order(:position) },

lib/alchemy/essence.rb

@@ -37,7 +37,7 @@ def acts_as_essence(options = {})
           stampable stamper_class_name: Alchemy.user_class_name
           validate :validate_ingredient, on: :update, if: -> { validations.any? }
 
-          has_one :content, as: :essence, class_name: "Alchemy::Content"
+          has_one :content, as: :essence, class_name: "Alchemy::Content", inverse_of: :essence
           has_one :element, through: :content, class_name: "Alchemy::Element"
           has_one :page,    through: :element, class_name: "Alchemy::Page"