PERF: 35% faster attributes for readonly usage

Instantiating attributes hash from raw database values is one of the
slower part of attributes.

Why that is necessary is to detect mutations. In other words, that isn't
necessary until mutations are happened.

`LazyAttributeHash` which was introduced at 0f29c21 is to instantiate
attribute lazily until first accessing the attribute (i.e.
`Model.find(1)` isn't slow yet, but `Model.find(1).attr_name` is still
slow).

This introduces `LazyAttributeSet` to instantiate attribute more lazily,
it doesn't instantiate attribute until first assigning/dirty checking
the attribute (i.e. `Model.find(1).attr_name` is no longer slow).

It makes attributes access about 35% faster for readonly (non-mutation)
usage.

https://gist.github.com/kamipo/4002c96a02859d8fe6503e26d7be4ad8

Before:

```
IPS
Warming up --------------------------------------
    attribute access     1.000  i/100ms
Calculating -------------------------------------
    attribute access      3.444  (± 0.0%) i/s -     18.000  in   5.259030s
MEMORY
Calculating -------------------------------------
    attribute access    38.902M memsize (     0.000  retained)
                       350.044k objects (     0.000  retained)
                        15.000  strings (     0.000  retained)
```

After (with `immutable_strings_by_default = true`):

```
IPS
Warming up --------------------------------------
    attribute access     1.000  i/100ms
Calculating -------------------------------------
    attribute access      4.652  (±21.5%) i/s -     23.000  in   5.034853s
MEMORY
Calculating -------------------------------------
    attribute access    27.782M memsize (     0.000  retained)
                       170.044k objects (     0.000  retained)
                        15.000  strings (     0.000  retained)
```

activemodel/lib/active_model/attribute.rb

@@ -5,16 +5,16 @@ require "active_support/core_ext/object/duplicable"
 module ActiveModel
   class Attribute # :nodoc:
     class << self
-      def from_database(name, value, type)
-        FromDatabase.new(name, value, type)
+      def from_database(name, value_before_type_cast, type, value = nil)
+        FromDatabase.new(name, value_before_type_cast, type, nil, value)
       end
 
-      def from_user(name, value, type, original_attribute = nil)
-        FromUser.new(name, value, type, original_attribute)
+      def from_user(name, value_before_type_cast, type, original_attribute = nil)
+        FromUser.new(name, value_before_type_cast, type, original_attribute)
       end
 
-      def with_cast_value(name, value, type)
-        WithCastValue.new(name, value, type)
+      def with_cast_value(name, value_before_type_cast, type)
+        WithCastValue.new(name, value_before_type_cast, type)
       end
 
       def null(name)
@@ -30,11 +30,12 @@ module ActiveModel
 
     # This method should not be called directly.
     # Use #from_database or #from_user
-    def initialize(name, value_before_type_cast, type, original_attribute = nil)
+    def initialize(name, value_before_type_cast, type, original_attribute = nil, value = nil)
       @name = name
       @value_before_type_cast = value_before_type_cast
       @type = type
       @original_attribute = original_attribute
+      @value = value unless value.nil?
     end
 
     def value

activemodel/lib/active_model/attribute_set/builder.rb

@@ -14,11 +14,17 @@ module ActiveModel
 
       def build_from_database(values = {}, additional_types = {})
         attributes = LazyAttributeHash.new(types, values, additional_types, default_attributes)
-        AttributeSet.new(attributes)
+        LazyAttributeSet.new(attributes)
       end
     end
   end
 
+  class LazyAttributeSet < AttributeSet # :nodoc:
+    def fetch_value(name, &block)
+      attributes.fetch_value(name, &block)
+    end
+  end
+
   class LazyAttributeHash # :nodoc:
     delegate :transform_values, :each_key, :each_value, :fetch, :except, to: :materialize
 
@@ -26,9 +32,10 @@ module ActiveModel
       @types = types
       @values = values
       @additional_types = additional_types
-      @materialized = false
-      @delegate_hash = delegate_hash
       @default_attributes = default_attributes
+      @delegate_hash = delegate_hash
+      @casted_values = {}
+      @materialized = false
     end
 
     def key?(key)
@@ -43,6 +50,25 @@ module ActiveModel
       delegate_hash[key] = value
     end
 
+    def fetch_value(name, &block)
+      if attr = delegate_hash[name]
+        return attr.value(&block)
+      end
+
+      @casted_values.fetch(name) do
+        value_present = true
+        value = values.fetch(name) { value_present = false }
+
+        if value_present
+          type = additional_types.fetch(name, types[name])
+          @casted_values[name] = type.deserialize(value)
+        else
+          attr = assign_default_value(name, value_present, value) || Attribute.null(name)
+          attr.value(&block)
+        end
+      end
+    end
+
     def deep_dup
       dup.tap do |copy|
         copy.instance_variable_set(:@delegate_hash, delegate_hash.transform_values(&:dup))
@@ -104,13 +130,15 @@ module ActiveModel
     private
       attr_reader :types, :values, :additional_types, :delegate_hash, :default_attributes
 
-      def assign_default_value(name)
-        type = additional_types.fetch(name, types[name])
-        value_present = true
+      def assign_default_value(
+        name,
+        value_present = true,
         value = values.fetch(name) { value_present = false }
+      )
+        type = additional_types.fetch(name, types[name])
 
         if value_present
-          delegate_hash[name] = Attribute.from_database(name, value, type)
+          delegate_hash[name] = Attribute.from_database(name, value, type, @casted_values[name])
         elsif types.key?(name)
           attr = default_attributes[name]
           if attr