Allow modification of data on [de]serialization

[altendky]

I was trying to implement a polyfield which turned {'a': 42} into {'a': {'type': 'int', 'value': 42} to make for extremely explicit type specification. This involves specifying a schema as already supported but also modifying the data both for serialization and deserialization.

The topic that lead me to work on this was exploring supporting more explicit specification for typing.Union in https://github.com/python-desert/desert.

[altendky]

Or maybe rather than supporting modification (along with supporting modification?) polyfield should configurably directly support explicit type naming? in the serialized data?