Make Iterator implementation of Deserializer::from_reader lazy

[Hubert Banaszewski]

The current implementation of Iterator for Deserializer created by from_reader constructor is eager. Upon calling next() for the first time, it uses Loader to load all the contents via read_to_end() on reader.

Such implementation is unfortunate for two reasons:

• it does not allow for online processing of documents on readers without EOF
• it wastes space and time, e.g. if someone needs to load only a single or few first documents from a large reader

I suggest loading only a single document from the reader at the time when calling next(). It could replace the current implementation or be optional feature accessible only by, for example, from_reader_lazy() constructor