proto: add IsInitialized

Move all checks for required fields into a proto.IsInitialized function.

Initial testing makes me confident that we can provide a fast-path
implementation of IsInitialized which will perform more than
acceptably.  (In the degenerate-but-common case where a message
transitively contains no required fields, this check can be nearly
zero cost.)

Unifying checks into a single function provides consistent behavior
between the wire, text, and json codecs.

Performing the check after decoding eliminates the wire decoder bug
where a split message is incorrectly seen as missing required fields.

Performing the check after decoding also provides consistent and
arguably more correct behavior when the target message was partially
prepopulated.

Change-Id: I9478b7bebb263af00c0d9f66a1f26e31ff553522
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/170787
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/encoding/textpb/encode.go b/encoding/textpb/encode.go
index 2ea370f..e293143 100644
--- a/encoding/textpb/encode.go
+++ b/encoding/textpb/encode.go
@@ -63,6 +63,9 @@
 	if !nerr.Merge(err) {
 		return nil, err
 	}
+	if !o.AllowPartial {
+		nerr.Merge(proto.IsInitialized(m))
+	}
 	return b, nerr.E
 }
 
@@ -91,10 +94,6 @@
 		num := fd.Number()
 
 		if !knownFields.Has(num) {
-			if !o.AllowPartial && fd.Cardinality() == pref.Required {
-				// Treat unset required fields as a non-fatal error.
-				nerr.AppendRequiredNotSet(string(fd.FullName()))
-			}
 			continue
 		}