explicit field order

[sunfish-shogi]

The filed order of struct is probably not guaranteed in future versions of Go compiler ???
https://stackoverflow.com/questions/32392311/reflect-type-field-order

If so, we should have order expressly.
For example:

type Emsg struct {
  FullBox               `mp4:"0,extend"`
  SchemeIdUri           string `mp4:"1,string"`
  Value                 string `mp4:"2,string"`
  Timescale             uint32 `mp4:"3,size=32"`
  PresentationTimeDelta uint32 `mp4:"4,size=32,ver=0"`
  PresentationTime      uint64 `mp4:"5,size=64,ver=1"`
  EventDuration         uint32 `mp4:"6,size=32"`
  Id                    uint32 `mp4:"7,size=32"`
  MessageData           []byte `mp4:"8,size=8,string"`
}

[sunfish-shogi]

related to:

• reflect: document struct field layout in memory golang/go#28989
• proposal: spec: define/imply memory layout of tagged struct fields golang/go#10014