Allow a field to register a "pack_later" function

[sprout42]

Some message fields, like a length_in_bytes field (added by #13), should be packed after the field(s) that it references. However, this means that it will need to somehow register a "later" function with the Message object so that when packing it is called after one or more fields are packed.

Perhaps this could be done when the Message object is being validated (in the way noted by #15), so the field can return a list of fields that should be packed before this element. Then the message that is being passed in the pack() function should either allow for an optional extra parameter which is the bytes of the following data (simple, but not quite as much validation as I prefer).

OR, perhaps the Message object could take the supplied dictionary or tuple, and create a new dictionary where the value for every field (key) is a tuple of (original value, bytes) and pass that to the pack() function for each field. That way the field pack could have the original data, AND it could have the bytes for fields which have already been packed. This way if the Message object changes the order that it calls the pack functions the elements themselves now have access to the actual packed data for referenced fields. This could be useful for future functionality, as well as for packing a "bytes length" field.