Stack-based unmarshaler (#546)

* Benchmark script

* Rewrite unmarshaler using the stack

Instead of tracking the build chain using `target`s, use the stack
instead.

Working and most benchmarks look good, but regression on structs unmarshalling.

~60% slower on ReferenceFile/struct.

* Shortcut to check if last node of iterator

* Remove unecessary pointer allocation

* Skip over unused keys without marking them as seen

* Add some tests

* Fix mktemp on macos

marshaler.go

@@ -640,8 +640,6 @@ func (enc *Encoder) encodeTableInline(b []byte, ctx encoderCtx, t table) ([]byte
 	return b, nil
 }
 
-var textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
-
 func willConvertToTable(ctx encoderCtx, v reflect.Value) bool {
 	if v.Type() == timeType || v.Type().Implements(textMarshalerType) {
 		return false