perf(json): address performance penalty found in v0.4.0+ (#149)

* perf(json): revert decodeInto helper changes for object/array/string

* per(json): add benchmarks for decoding object and array with mixed members

Author: dominicbarnes

json/decode.go

@@ -1292,15 +1292,19 @@ func (d decoder) decodeInterface(b []byte, p unsafe.Pointer) ([]byte, error) {
 
 	switch k.Class() {
 	case Object:
-		v, err = decodeInto[map[string]any](&val, v, d, decoder.decodeMapStringInterface)
+		m := make(map[string]interface{})
+		v, err = d.decodeMapStringInterface(v, unsafe.Pointer(&m))
+		val = m
 
 	case Array:
-		size := alignedSize(interfaceType)
-		fn := constructSliceDecodeFunc(size, sliceInterfaceType, decoder.decodeInterface)
-		v, err = decodeInto[[]any](&val, v, d, fn)
+		a := make([]interface{}, 0, 10)
+		v, err = d.decodeSlice(v, unsafe.Pointer(&a), unsafe.Sizeof(a[0]), sliceInterfaceType, decoder.decodeInterface)
+		val = a
 
 	case String:
-		v, err = decodeInto[string](&val, v, d, decoder.decodeString)
+		s := ""
+		v, err = d.decodeString(v, unsafe.Pointer(&s))
+		val = s
 
 	case Null:
 		v, val = nil, nil

json/golang_bench_test.go

@@ -261,6 +261,32 @@ func BenchmarkUnmarshalString(b *testing.B) {
 	})
 }
 
+func BenchmarkUnmarshalObjectMixed(b *testing.B) {
+	b.ReportAllocs()
+	data := []byte(`{"string":"hello world","time":"2025-07-17T18:40:04.338Z","bool":true,"integer":42,"decimal":3.14,"null":null,"object":{"hello":"world"},"array":[1,2,3]}`)
+	b.RunParallel(func(pb *testing.PB) {
+		var m map[string]any
+		for pb.Next() {
+			if err := Unmarshal(data, &m); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
+func BenchmarkUnmarshalArrayMixed(b *testing.B) {
+	b.ReportAllocs()
+	data := []byte(`["hello world","2025-07-17T18:40:04.338Z",true,42,3.14,null,{"hello":"world"},[1,2,3]]`)
+	b.RunParallel(func(pb *testing.PB) {
+		var a []any
+		for pb.Next() {
+			if err := Unmarshal(data, &a); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
 func BenchmarkUnmarshalFloat64(b *testing.B) {
 	b.ReportAllocs()
 	data := []byte(`3.14`)