Ensure uctypes data isn't GC'd prematurely

[jepler]

MicroPython's GC requires that a pointer to the beginning of an allocated chunk has to be kept somewhere in another reachable object, or the chunk can be freed when GC happens.

However, with uctypes nested structures, this was not necessarily the case. For example, before this PR, you could lose the contents of a pnSize object:

MicroPython 20250717-9.g4a46582605.dirty on 2025-07-20; macplus with m68000
Type "help()" for more information.
>>> import qd, gc
>>> sz = qd.PenState().pnSize # Sub-field of PenState not at start
>>> sz.h = 8
>>> sz.v = 12
>>> import gc
>>> gc.collect()
>>> for _ in range(100): l = [] # Make a bunch of objects
>>> (sz.h, sz.v)
(4, 0)   # Because the storage for PenState was used for other objects

After this PR, the correct value of (8, 12) is printed.