Abandon the `_fields_` syntax in lieu of defining class-level properties

[kendas]

The preservation of order should be possible. This behavior can be observed in Django's models.

The new format should change from this:

class DefaultValuePacket(SerdepaPacket):
    _fields_ = [
        ("header", nx_uint8, 1),
        ("version", nx_uint8),
        ("timestamp", nx_uint32, 12345),
        ("length", Length(nx_uint8, "data")),
        ("data", List(nx_uint8), [1, 2, 3, 4]),
        ("tail", List(nx_uint8), [5, 6])
]

to this:

class DefaultValuePacket(SerdepaPacket):
    header = nx_uint8(1)
    version = nx_uint8
    timestamp = nx_uint32(12345)
    length = Length(nx_uint8, "data")
    data = List(nx_uint8, [1, 2, 3, 4]),
    tail = List(nx_uint8, [5, 6])

[kendas]

@raidoz

Upon review, this seems impossible prior to Python 3.0, where the __prepare__ method of a metaclass provides the ability to track the order of attributes (PEP 3115). In Python 3.6, this was further improved to provide the __dict__ as an ordered dictionary (PEP 520).

Seeing as we have no plans of abandoning Python 2 support in the foreseeable future, this feature could be implemented on only Python 3 or not at all.

[kendas]

This approach could be interesting for Python 2: http://numberoverzero.com/blog/2013/10/07/maintaining-class-attribute-order.html