runtime/protoiface: move and rename XXX_Methods

This CL moves and renames the protoreflect.ProtoMessage.XXX_Methods
to protoreflect.Message.ProtoMethods.

Since one needs to obtain a protoreflect.Message now to get at
the fast-path methods, we modify the method signatures to take
in a protoreflect.Message instead of protoreflect.ProtoMessage.
Doing so also avoids the wrapper hack that was formerly done on
impl.messageReflectWrapper.

After this change the new protoc-gen-go no longer generates
any XXX fields or methods. All internal fields and methods are truly
hidden from the end-user.

name                                     old time/op    new time/op    delta
Wire/Unmarshal/google_message1_proto2-4    1.50µs ±10%    1.50µs ± 2%    ~     (p=0.483 n=10+9)
Wire/Unmarshal/google_message1_proto3-4    1.06µs ± 6%    1.06µs ± 4%    ~     (p=0.814 n=9+9)
Wire/Unmarshal/google_message2-4            734µs ±22%     689µs ±13%    ~     (p=0.133 n=10+9)
Wire/Marshal/google_message1_proto2-4       790ns ±46%     652ns ± 8%    ~     (p=0.590 n=10+9)
Wire/Marshal/google_message1_proto3-4       872ns ± 4%     857ns ± 3%    ~     (p=0.168 n=9+9)
Wire/Marshal/google_message2-4              232µs ±16%     221µs ± 3%  -4.75%  (p=0.014 n=9+9)
Wire/Size/google_message1_proto2-4          164ns ± 2%     167ns ± 4%  +1.87%  (p=0.046 n=9+10)
Wire/Size/google_message1_proto3-4          240ns ± 9%     229ns ± 1%  -4.81%  (p=0.000 n=9+8)
Wire/Size/google_message2-4                58.9µs ± 9%    59.6µs ± 2%  +1.23%  (p=0.040 n=9+9)

name                                     old alloc/op   new alloc/op   delta
Wire/Unmarshal/google_message1_proto2-4      912B ± 0%      912B ± 0%    ~     (all equal)
Wire/Unmarshal/google_message1_proto3-4      688B ± 0%      688B ± 0%    ~     (all equal)
Wire/Unmarshal/google_message2-4            470kB ± 0%     470kB ± 0%    ~     (p=0.215 n=10+10)
Wire/Marshal/google_message1_proto2-4        240B ± 0%      240B ± 0%    ~     (all equal)
Wire/Marshal/google_message1_proto3-4        224B ± 0%      224B ± 0%    ~     (all equal)
Wire/Marshal/google_message2-4             90.1kB ± 0%    90.1kB ± 0%    ~     (all equal)
Wire/Size/google_message1_proto2-4          0.00B          0.00B         ~     (all equal)
Wire/Size/google_message1_proto3-4          0.00B          0.00B         ~     (all equal)
Wire/Size/google_message2-4                 0.00B          0.00B         ~     (all equal)

name                                     old allocs/op  new allocs/op  delta
Wire/Unmarshal/google_message1_proto2-4      24.0 ± 0%      24.0 ± 0%    ~     (all equal)
Wire/Unmarshal/google_message1_proto3-4      6.00 ± 0%      6.00 ± 0%    ~     (all equal)
Wire/Unmarshal/google_message2-4            8.49k ± 0%     8.49k ± 0%    ~     (all equal)
Wire/Marshal/google_message1_proto2-4        1.00 ± 0%      1.00 ± 0%    ~     (all equal)
Wire/Marshal/google_message1_proto3-4        1.00 ± 0%      1.00 ± 0%    ~     (all equal)
Wire/Marshal/google_message2-4               1.00 ± 0%      1.00 ± 0%    ~     (all equal)
Wire/Size/google_message1_proto2-4           0.00           0.00         ~     (all equal)
Wire/Size/google_message1_proto3-4           0.00           0.00         ~     (all equal)
Wire/Size/google_message2-4                  0.00           0.00         ~     (all equal)

Change-Id: Ibf3263ad0f293326695c22020a92a6b938ef4f65
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185697
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect_gen.go b/internal/impl/message_reflect_gen.go
index 41ba0b9..efe4e89 100644
--- a/internal/impl/message_reflect_gen.go
+++ b/internal/impl/message_reflect_gen.go
@@ -8,6 +8,7 @@
 
 import (
 	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/runtime/protoiface"
 )
 
 func (m *messageState) Descriptor() protoreflect.MessageDescriptor {
@@ -22,6 +23,10 @@
 func (m *messageState) ProtoUnwrap() interface{} {
 	return m.pointer().AsIfaceOf(m.mi.GoType.Elem())
 }
+func (m *messageState) ProtoMethods() *protoiface.Methods {
+	m.mi.init()
+	return &m.mi.methods
+}
 
 func (m *messageState) Range(f func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
 	m.mi.init()
@@ -113,6 +118,10 @@
 func (m *messageReflectWrapper) ProtoUnwrap() interface{} {
 	return m.pointer().AsIfaceOf(m.mi.GoType.Elem())
 }
+func (m *messageReflectWrapper) ProtoMethods() *protoiface.Methods {
+	m.mi.init()
+	return &m.mi.methods
+}
 
 func (m *messageReflectWrapper) Range(f func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
 	m.mi.init()