Make SendNode#macro? aware of struct constructor

This PR makes `SendNode#macro?` aware of struct constructor.

Like class constructor, struct constructor will be recognized as a
macro and will be awakened in the following `private` modifier:

```ruby
Foo = Struct.new(:foo) do
  private
    def private_foo
      foo
    end
end
```

With this change, this PR aims to resolve the following issue.
rubocop/rubocop#8919

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Changes
+
+* [#141](https://github.com/rubocop-hq/rubocop-ast/pull/141): Make `SendNode#macro?` aware of struct constructor. ([@koic][])
+
 ## 1.0.0 (2020-10-21)
 
 ### Changes

lib/rubocop/ast/node/mixin/method_dispatch_node.rb

@@ -229,6 +229,7 @@ def binary_operation?
           root?                                    # Either a root node,
           ^{                                       # or the parent is...
             sclass class module class_constructor? # a class-like node
+            struct_constructor?
             [ {                                    # or some "wrapper"
                 kwbegin begin block
                 (if _condition <%0 _>)  # note: we're excluding the condition of `if` nodes

spec/rubocop/ast/send_node_spec.rb

@@ -242,6 +242,17 @@ module Foo
         it { expect(send_node).to be_macro }
       end
 
+      context 'when parent is a struct constructor' do
+        let(:source) do
+          ['Foo = Struct.new do',
+           '>>bar :baz<<',
+           '  bar :qux',
+           'end'].join("\n")
+        end
+
+        it { expect(send_node).to be_macro }
+      end
+
       context 'when parent is a singleton class' do
         let(:source) do
           ['class << self',