Propagate parse errors inside a flattened Option struct

[johnmave126]

This is an attempt to address serde-rs/json#644

The idea is to go through a proxy type OptionFlatMapDeserializer when calling deserialize_option in FlatMapDeserializer. Such proxy type provides a custom error type FlatStructError which only catches missing_field error during the immediate struct deserialization. All the errors happening nested will be preserved in another variant. In this way, we could propagate error inside a flattened struct.

I guess some more tests and comments are needed, but I want to put this work forward for preliminary test.