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/runtime/protoiface/methods.go b/runtime/protoiface/methods.go
index e4a5157..eb6c5f3 100644
--- a/runtime/protoiface/methods.go
+++ b/runtime/protoiface/methods.go
@@ -15,10 +15,10 @@
 	"google.golang.org/protobuf/reflect/protoregistry"
 )
 
-// Methoder is an optional interface implemented by generated messages to
+// Methoder is an optional interface implemented by protoreflect.Message to
 // provide fast-path implementations of various operations.
 type Methoder interface {
-	XXX_Methods() *Methods // may return nil
+	ProtoMethods() *Methods // may return nil
 }
 
 // Methods is a set of optional fast-path implementations of various operations.
@@ -28,17 +28,17 @@
 
 	// MarshalAppend appends the wire-format encoding of m to b, returning the result.
 	// It does not perform required field checks.
-	MarshalAppend func(b []byte, m protoreflect.ProtoMessage, opts MarshalOptions) ([]byte, error)
+	MarshalAppend func(b []byte, m protoreflect.Message, opts MarshalOptions) ([]byte, error)
 
 	// Size returns the size in bytes of the wire-format encoding of m.
-	Size func(m protoreflect.ProtoMessage) int
+	Size func(m protoreflect.Message) int
 
 	// Unmarshal parses the wire-format message in b and places the result in m.
 	// It does not reset m or perform required field checks.
-	Unmarshal func(b []byte, m protoreflect.ProtoMessage, opts UnmarshalOptions) error
+	Unmarshal func(b []byte, m protoreflect.Message, opts UnmarshalOptions) error
 
 	// IsInitialized returns an error if any required fields in m are not set.
-	IsInitialized func(m protoreflect.ProtoMessage) error
+	IsInitialized func(m protoreflect.Message) error
 
 	pragma.NoUnkeyedLiterals
 }