model: improve the core model to allow recursive object structures

model: improve the core model to allow recursive object structures

Previously, the logic for serializing/deserializing to/from dictionaries
did not properly deal with lists of hashes, hashes of lists, lists, etc.

The core model can now handle any arbitrarily complicated structure.
The core to_dict() routine recursively walks the object structure
without the need for external data for tracking parameter types.

Signed-off-by: David Aguilar <davvid@gmail.com>