Generate "Decode" for "enums" that will directly decode

Add templating for generating code for "enums" that will utilize the new
`stream.Reader`, performing the same duties as `binary.Decode` and `FromWire`,
but without going through an intermediary format.

gen/enum.go

@@ -69,6 +69,7 @@ func enum(g Generator, spec *compile.EnumSpec) error {
 		<$strconv := import "strconv">
 
 		<$wire := import "go.uber.org/thriftrw/wire">
+		<$stream := import "go.uber.org/thriftrw/protocol/stream">
 
 		<$enumName := goName .Spec>
 		<formatDoc .Spec.Doc>type <$enumName> int32
@@ -190,6 +191,26 @@ func enum(g Generator, spec *compile.EnumSpec) error {
 			return nil
 		}
 
+		<$sr := newVar "sr">
+		// Decode reads off the encoded <$enumName> directly off of the wire.
+		//
+		//   sReader := BinaryStreamer.Reader(reader)
+		//
+		//   var <$v> <$enumName>
+		//   if err := <$v>.Decode(sReader); err != nil {
+		//     return <$enumName>(0), err
+		//   }
+		//   return <$v>, nil
+		func (<$v> *<$enumName>) Decode(<$sr> <$stream>.Reader) error {
+			<- $i := newVar "i" ->
+			<$i>, err := <$sr>.ReadInt32()
+			if err != nil {
+				return err
+			}
+			*<$v> = (<$enumName>)(<$i>)
+			return nil
+		}
+
 		// String returns a readable string representation of <$enumName>.
 		func (<$v> <$enumName>) String() string {
 			<$w> := int32(<$v>)

gen/enum_test.go

@@ -108,6 +108,7 @@ func TestEnumDefaultWire(t *testing.T) {
 
 	for _, tt := range tests {
 		assertRoundTrip(t, &tt.e, tt.v, "EnumDefault")
+		assertStreamingRoundTrip(t, &tt.e, tt.v, "EnumDefault")
 	}
 }
 
@@ -154,6 +155,7 @@ func TestEnumWithDuplicateValuesWire(t *testing.T) {
 
 	for _, tt := range tests {
 		assertRoundTrip(t, &tt.e, tt.v, "EnumWithDuplicateValues")
+		assertStreamingRoundTrip(t, &tt.e, tt.v, "EnumWithDuplicateValues")
 	}
 }
 
@@ -496,6 +498,9 @@ func TestEnumLabelValid(t *testing.T) {
 			t.Run("wire", func(t *testing.T) {
 				assertRoundTrip(t, &tt.item, wire.NewValueI32(int32(tt.item)),
 					"%v", tt.item)
+				assertStreamingRoundTrip(t, &tt.item, wire.NewValueI32(int32(tt.item)),
+					"%v", tt.item)
+
 			})
 		})
 	}