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/proto/encode.go b/proto/encode.go
index 5139cce..f9a753b 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -87,10 +87,7 @@
 	// effort with the single initialization check below.
 	allowPartial := o.AllowPartial
 	o.AllowPartial = true
-	out, err := o.marshalMessageFast(b, m)
-	if err == errInternalNoFast {
-		out, err = o.marshalMessage(b, m.ProtoReflect())
-	}
+	out, err := o.marshalMessage(b, m.ProtoReflect())
 	if err != nil {
 		return nil, err
 	}
@@ -100,26 +97,24 @@
 	return out, IsInitialized(m)
 }
 
-func (o MarshalOptions) marshalMessageFast(b []byte, m Message) ([]byte, error) {
-	methods := protoMethods(m)
-	if methods == nil ||
-		methods.MarshalAppend == nil ||
-		(o.Deterministic && methods.Flags&protoiface.MethodFlagDeterministicMarshal == 0) {
-		return nil, errInternalNoFast
-	}
-	if methods.Size != nil {
-		sz := methods.Size(m)
-		if cap(b) < len(b)+sz {
-			x := make([]byte, len(b), len(b)+sz)
-			copy(x, b)
-			b = x
+func (o MarshalOptions) marshalMessage(b []byte, m protoreflect.Message) ([]byte, error) {
+	if methods := protoMethods(m); methods != nil && methods.MarshalAppend != nil &&
+		!(o.Deterministic && methods.Flags&protoiface.MethodFlagDeterministicMarshal == 0) {
+		if methods.Size != nil {
+			sz := methods.Size(m)
+			if cap(b) < len(b)+sz {
+				x := make([]byte, len(b), len(b)+sz)
+				copy(x, b)
+				b = x
+			}
+			o.UseCachedSize = true
 		}
-		o.UseCachedSize = true
+		return methods.MarshalAppend(b, m, protoiface.MarshalOptions(o))
 	}
-	return methods.MarshalAppend(b, m, protoiface.MarshalOptions(o))
+	return o.marshalMessageSlow(b, m)
 }
 
-func (o MarshalOptions) marshalMessage(b []byte, m protoreflect.Message) ([]byte, error) {
+func (o MarshalOptions) marshalMessageSlow(b []byte, m protoreflect.Message) ([]byte, error) {
 	// There are many choices for what order we visit fields in. The default one here
 	// is chosen for reasonable efficiency and simplicity given the protoreflect API.
 	// It is not deterministic, since Message.Range does not return fields in any