Remove useless set_value / get_value helper methods

Those helper methods makes relation values access 15% slower.

https://gist.github.com/kamipo/e64439f7a206e1c5b5c69d92d982828e

Before (02b5b8c):

```
Warming up --------------------------------------
        #limit_value   237.074k i/100ms
    #limit_value = 1   222.052k i/100ms
Calculating -------------------------------------
        #limit_value      6.477M (± 2.9%) i/s -     32.479M in   5.019475s
    #limit_value = 1      5.297M (± 4.3%) i/s -     26.424M in   4.999933s
```

After (this change):

```
Warming up --------------------------------------
        #limit_value   261.109k i/100ms
    #limit_value = 1   239.646k i/100ms
Calculating -------------------------------------
        #limit_value      7.412M (± 1.6%) i/s -     37.077M in   5.003345s
    #limit_value = 1      6.134M (± 1.0%) i/s -     30.675M in   5.000908s
```

activerecord/lib/active_record/relation.rb

@@ -553,8 +553,8 @@ def destroy_all
     #   # => ActiveRecord::ActiveRecordError: delete_all doesn't support distinct
     def delete_all
       invalid_methods = INVALID_METHODS_FOR_DELETE_ALL.select do |method|
-        value = get_value(method)
-        SINGLE_VALUE_METHODS.include?(method) ? value : value.any?
+        value = @values[method]
+        method == :distinct ? value : value&.any?
       end
       if invalid_methods.any?
         raise ActiveRecordError.new("delete_all doesn't support #{invalid_methods.join(', ')}")

activerecord/lib/active_record/relation/query_methods.rb

@@ -67,11 +67,13 @@ def not(opts, *rest)
         end
       class_eval <<-CODE, __FILE__, __LINE__ + 1
         def #{method_name}                   # def includes_values
-          get_value(#{name.inspect})         #   get_value(:includes)
+          default = DEFAULT_VALUES[:#{name}] #   default = DEFAULT_VALUES[:includes]
+          @values.fetch(:#{name}, default)   #   @values.fetch(:includes, default)
         end                                  # end
 
         def #{method_name}=(value)           # def includes_values=(value)
-          set_value(#{name.inspect}, value)  #   set_value(:includes, value)
+          assert_mutability!                 #   assert_mutability!
+          @values[:#{name}] = value          #   @values[:includes] = value
         end                                  # end
       CODE
     end
@@ -417,7 +419,8 @@ def unscope!(*args) # :nodoc:
           if !VALID_UNSCOPING_VALUES.include?(scope)
             raise ArgumentError, "Called unscope() with invalid unscoping argument ':#{scope}'. Valid arguments are :#{VALID_UNSCOPING_VALUES.to_a.join(", :")}."
           end
-          set_value(scope, DEFAULT_VALUES[scope])
+          assert_mutability!
+          @values[scope] = DEFAULT_VALUES[scope]
         when Hash
           scope.each do |key, target_value|
             if key != :where
@@ -1005,17 +1008,6 @@ def build_subquery(subquery_alias, select_value) # :nodoc:
       end
 
     private
-      # Returns a relation value with a given name
-      def get_value(name)
-        @values.fetch(name, DEFAULT_VALUES[name])
-      end
-
-      # Sets the relation value with the given name
-      def set_value(name, value)
-        assert_mutability!
-        @values[name] = value
-      end
-
       def assert_mutability!
         raise ImmutableRelation if @loaded
         raise ImmutableRelation if defined?(@arel) && @arel
@@ -1316,7 +1308,8 @@ def check_if_method_has_arguments!(method_name, args)
       def structurally_incompatible_values_for_or(other)
         values = other.values
         STRUCTURAL_OR_METHODS.reject do |method|
-          get_value(method) == values.fetch(method, DEFAULT_VALUES[method])
+          default = DEFAULT_VALUES[method]
+          @values.fetch(method, default) == values.fetch(method, default)
         end
       end