Most symbols appear directly in prelude without modules.
There is 'user_data' nested, as it's used rarely and helps organize its types.

Changes:
* Move out symbols from export::{method, property}
* Flatten property::accessor::invalid
* Rename Usage -> PropertyUsage (like the already re-exported version)

…itional types (not the API class itself)

…e empty nested modules

Removed:
* godot_gdnative_init
* godot_gdnative_terminate
* godot_nativescript_init
* godot_site

Also documented prelude::user_data