Call "Class#new" over rb_class_new_instance in decoding (#7352)

This patch has almost no change in behaviour where users have not
patched the implementation of new on either a specific proto object
class, or `Google::Protobuf::MessageExts::ClassMethods`. The default
implementation of `new`, and `rb_class_new_instance` have the same
behaviour.

By default when we call `new` on a class in Ruby, it goes to the `Class`
class's implementation:

```ruby
class Foo
end

>> Foo.method(:new).owner
=> Class
```

the `Class` implementation of `new` is (pseudocode, it's actually in c):

```ruby
class Class
  def new(*args, &blk)
    instance = alloc
    instance.initialize(*args, &blk)
    instance
  end
end
```

`rb_class_new_instance` does the same thing, it calls down to
[`rb_class_s_new`](https://github.com/ruby/ruby/blob/v2_5_5/object.c#L2147),
which calls `rb_class_alloc`, then `rb_obj_call_init`.

`rb_funcall` is a variadic c function for calling a ruby method on an object,
it takes:

* A `VALUE` on to which the method should be called
* An `ID` which should be an ID of a method, usually created with `rb_intern`,
  to get an ID from a string
* An integer, the number of arguments calling the  method with,
* A number of `VALUE`s, to send to the method call.

`rb_funcall` is the same as calling a method directly in Ruby, and will perform
ancestor chain respecting method lookup.

This means that in C extensions, if nobody has defined the `new` method on any
classes or modules in a class's inheritance chain calling
`rb_class_new_instance` is the same as calling `rb_funcall(klass,
rb_intern("new"))`, *however* this removes the ability for users to define or
monkey patch their own constructors in to the objects created by the C
extension.

In Ads, we define [`new`](https://git.io/JvFC9) on
`Google::Protobuf::MessageExts::ClassMethods` to allow us to insert a
monkeypatch which makes it possible to assign primitive values to wrapper type
fields (e.g. Google::Protobuf::StringValue). The monkeypatch we apply works for
objects that we create for the user via the `new` method. Before this commit,
however, the patch does not work for the `decode` method, for the reasons
outlined above. Before this commit, protobuf uses `rb_class_new_instance`.

After this commit, we use `rb_funcall(klass, rb_intern("new"), 0);` to construct
protobuf objects during decoding. While I haven't measured it this will have
a very minor performance impact for decoding (`rb_funcall` will have to go to the
method cache, which `rb_class_new_instance` will not). This does however do
the "more rubyish" thing of respecting the protobuf object's inheritance chain
to construct them during decoding.

I have run both Ads and Cloud's test suites for Ruby libraries against this
patch, as well as the protobuf Ruby gem's test suite locally.

ruby/ext/google/protobuf_c/encode_decode.c

@@ -30,6 +30,10 @@
 
 #include "protobuf.h"
 
+VALUE initialize_rb_class_with_no_args(VALUE klass) {
+  return rb_funcall(klass, rb_intern("new"), 0);
+}
+
 // This function is equivalent to rb_str_cat(), but unlike the real
 // rb_str_cat(), it doesn't leak memory in some versions of Ruby.
 // For more information, see:
@@ -295,7 +299,7 @@ static void *appendsubmsg_handler(void *closure, const void *hd) {
   const submsg_handlerdata_t *submsgdata = hd;
   MessageHeader* submsg;
 
-  VALUE submsg_rb = rb_class_new_instance(0, NULL, submsgdata->subklass);
+  VALUE submsg_rb = initialize_rb_class_with_no_args(submsgdata->subklass);
   RepeatedField_push(ary, submsg_rb);
 
   TypedData_Get_Struct(submsg_rb, MessageHeader, &Message_type, submsg);
@@ -322,7 +326,7 @@ static void *submsg_handler(void *closure, const void *hd) {
 
   if (DEREF(msg, submsgdata->ofs, VALUE) == Qnil) {
     DEREF(msg, submsgdata->ofs, VALUE) =
-        rb_class_new_instance(0, NULL, submsgdata->subklass);
+        initialize_rb_class_with_no_args(submsgdata->subklass);
   }
 
   set_hasbit(closure, submsgdata->hasbit);
@@ -549,7 +553,7 @@ static void *oneofsubmsg_handler(void *closure,
   if (oldcase != oneofdata->oneof_case_num ||
       DEREF(msg, oneofdata->ofs, VALUE) == Qnil) {
     DEREF(msg, oneofdata->ofs, VALUE) =
-        rb_class_new_instance(0, NULL, oneofdata->subklass);
+        initialize_rb_class_with_no_args(oneofdata->subklass);
   }
   // Set the oneof case *after* allocating the new class instance -- otherwise,
   // if the Ruby GC is invoked as part of a call into the VM, it might invoke
@@ -1038,7 +1042,7 @@ VALUE Message_decode(VALUE klass, VALUE data) {
     rb_raise(rb_eArgError, "Expected string for binary protobuf data.");
   }
 
-  msg_rb = rb_class_new_instance(0, NULL, msgklass);
+  msg_rb = initialize_rb_class_with_no_args(msgklass);
   TypedData_Get_Struct(msg_rb, MessageHeader, &Message_type, msg);
 
   {
@@ -1114,7 +1118,7 @@ VALUE Message_decode_json(int argc, VALUE* argv, VALUE klass) {
   // convert, because string handlers pass data directly to message string
   // fields.
 
-  msg_rb = rb_class_new_instance(0, NULL, msgklass);
+  msg_rb = initialize_rb_class_with_no_args(msgklass);
   TypedData_Get_Struct(msg_rb, MessageHeader, &Message_type, msg);
 
   {