reflect/protoreflect: rename methods with Type suffix

The protobuf type system uses the word "descriptor" instead of "type".
We should avoid the "type" verbage when we aren't talking about Go types.
The old names are temporarily kept around for compatibility reasons.

Change-Id: Icc99c913528ead011f7a74aa8399d9c5ec6dc56e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172238
Reviewed-by: Herbie Ong <herbie@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/proto/decode.go b/proto/decode.go
index 0b1aa3f..5a867a2 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -118,7 +118,7 @@
 		// TODO: C++ merges into oneofs, while v1 does not.
 		// Evaluate which behavior to pick.
 		var m protoreflect.Message
-		if knownFields.Has(num) && field.OneofType() == nil {
+		if knownFields.Has(num) && field.Oneof() == nil {
 			m = knownFields.Get(num).Message()
 		} else {
 			m = knownFields.NewMessage(num)
@@ -144,8 +144,8 @@
 		return 0, wire.ParseError(n)
 	}
 	var (
-		keyField = field.MessageType().Fields().ByNumber(1)
-		valField = field.MessageType().Fields().ByNumber(2)
+		keyField = field.Message().Fields().ByNumber(1)
+		valField = field.Message().Fields().ByNumber(2)
 		key      protoreflect.Value
 		val      protoreflect.Value
 		haveKey  bool
diff --git a/proto/encode.go b/proto/encode.go
index 8635790..1b48b41 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -184,7 +184,7 @@
 		b = wire.AppendTag(b, num, wireTypes[kind])
 		return o.marshalSingular(b, num, field, value)
 	case field.IsMap():
-		return o.marshalMap(b, num, kind, field.MessageType(), value.Map())
+		return o.marshalMap(b, num, kind, field.Message(), value.Map())
 	case field.IsPacked():
 		return o.marshalPacked(b, num, field, value.List())
 	default:
diff --git a/proto/isinit.go b/proto/isinit.go
index 33dfb64..08edbd8 100644
--- a/proto/isinit.go
+++ b/proto/isinit.go
@@ -48,12 +48,12 @@
 		}
 		// Look for fields containing a message: Messages, groups, and maps
 		// with a message or group value.
-		ft := field.MessageType()
-		if ft == nil {
+		md := field.Message()
+		if md == nil {
 			return true
 		}
 		if field.IsMap() {
-			if ft.Fields().ByNumber(2).MessageType() == nil {
+			if md.Fields().ByNumber(2).Message() == nil {
 				return true
 			}
 		}
diff --git a/proto/size.go b/proto/size.go
index ab21cb3..3bc5a3c 100644
--- a/proto/size.go
+++ b/proto/size.go
@@ -61,7 +61,7 @@
 	case field.Cardinality() != protoreflect.Repeated:
 		return wire.SizeTag(num) + sizeSingular(num, kind, value)
 	case field.IsMap():
-		return sizeMap(num, kind, field.MessageType(), value.Map())
+		return sizeMap(num, kind, field.Message(), value.Map())
 	case field.IsPacked():
 		return sizePacked(num, kind, value.List())
 	default: