fix: rewrite __class__ closure cells of methods hidden in decorators

[TrueFurina]

When a slotted class is re-created by _ClassBuilder, closure cells that reference the old class (baked in by no-arg super() or class) are rewritten to point at the new class. Methods wrapped in decorators hide their function - and thus the class cell baked into it - behind the wrapper's own closure, so those cells were never rewritten. This caused no-arg super() inside such wrapped methods to raise:

TypeError: super(type, obj): obj is not an instance or subtype of type

Collect functions referenced by other functions' closures as well, so their cells are rewritten too. Fixes #1038.

Summary

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