Preserve default factories when pickling munches

[vitalivo]

This replaces #109, which was accidentally closed and its source fork deleted. The implementation is unchanged; the original discussion and reviews remain linked there.

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Pickling a DefaultFactoryMunch(list, existing=[1]) currently discards default_factory. Accessing a missing attribute on the restored object then raises RecursionError, because looking up the missing factory invokes __missing__ again.

Store the factory alongside the dictionary state and restore it before the items. This also preserves nested RecursiveMunch behavior. Regression tests cover all six pickle protocols, independent default values, and recursive factories.

Validation: 122 tests pass on Python 3.11 with PyYAML. The six original factory regressions failed before the fix. The optional module doctests have four pre-existing failures (dictionary ordering, multiline output, and the old yaml.load call); the same four failures were verified on unchanged HEAD.