Fix T86332: Error using lambda in annotations in Python 3.10

Callbacks used in `bpy.props` didn't hold a references to the functions
they used.

While this has been the case since early 2.5x it didn't cause any
problems as long as the class held a reference.

With Python 3.10 or when using `from __future__ import annotations`,
the annotations are no longer owned by the class once evaluated.

Resolve this by holding a reference in the module, which now supports
traverse & clear callbacks so the objects are visible to Python's
garbage collector.

Also refactor storage of Python data, moving from an array into a struct.

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