clearer alternative to loading_fn

[mivanit]

serializeable_field() currently takes a loading_fn parameter, which, due to a dumb choice by me ages ago, takes directly the dict representing the whole object instead of the actual field we care about. this makes code a bit messy. It was originally to allow for extracting data from other fields in the object, but this is bad practice. Currently, the syntax looks like:

@serializable_dataclass
class MyClass(SerializableDataclass):
  some_field: int = serializable_field(
    serialization_fn = lambda x : str(x),
    loading_fn = lambda x : int(x["some_field"])
  )

Changing the behavior of the existing function would probably be a bad idea, so I think the best course of action is to create a new parameter, perhaps deserialization_fn, which allows us to do

deserialization_fn = lambda x : int(x)

This should be a change of just a few lines in muutils.json_serialize.serializable_dataclass