perf(encode): add map[string]string fast path in Encode() type switch

Adds a direct type assertion case for map[string]string in Encode(),
bypassing reflect.ValueOf + getEncoder dispatch. Also adds
encodeMapStringString method that operates on the concrete type
without reflection.

Benchmark: MapStringString 192 ns -> 164 ns (-15%)

Author: Ignacio Van Droogenbroeck

encode.go

@@ -243,6 +243,8 @@ func (e *Encoder) Encode(v interface{}) error {
 		return e.encodeInt64Cond(int64(v))
 	case time.Time:
 		return e.EncodeTime(v)
+	case map[string]string:
+		return e.encodeMapStringString(v)
 	case map[string]interface{}:
 		if e.flags&sortMapKeysFlag != 0 {
 			return e.EncodeMapSorted(v)

encode_map.go

@@ -93,6 +93,27 @@ func encodeMapStringInterfaceValue(e *Encoder, v reflect.Value) error {
 	return e.EncodeMap(m)
 }
 
+func (e *Encoder) encodeMapStringString(m map[string]string) error {
+	if m == nil {
+		return e.EncodeNil()
+	}
+	if err := e.EncodeMapLen(len(m)); err != nil {
+		return err
+	}
+	if e.flags&sortMapKeysFlag != 0 {
+		return e.encodeSortedMapStringString(m)
+	}
+	for mk, mv := range m {
+		if err := e.EncodeString(mk); err != nil {
+			return err
+		}
+		if err := e.EncodeString(mv); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (e *Encoder) EncodeMap(m map[string]interface{}) error {
 	if m == nil {
 		return e.EncodeNil()