Fix handling of complex nested definitions

[mikee47]

This PR aims to resolve the issues raised in #50.

Verify use of store annotation

Must only be present on a root object. Code generator raises an exception if, for example, this annotation is found on an array.

Fix type information namespace

This is incorrectly generated for nested object definitions.

Nested objects missing default constructors

Required for exception conditions, such as accessing an invalid object type on a Union.

Code generator confuses is_store and is_root

is_root and is_store both return True for children of database.
This messes up global definitions.
Use a member variable for is_store and fix is_root implementation.

Complex nested objects require (object, prop, offset) constructor

This is required so a Union can return the embedded object type.