…partially initialised variables (Fixes python#12915).

This proposal is an alternative to python#14423.  Similar to python#14423, the main idea is to convert unions of tuples to tuples of (simplified) unions during multi-assignment checks.  In addition, it extends this idea to other iterable types, which allows removing the `undefined_rvalue` logic and the `no_partial_types` logic.  Hence, the problem reported in python#12915 with partially initialised variables should be fixed for unions that combine, for example, tuples and lists, as well.

Besides the new test case also provided by python#14423 (`testDefinePartiallyInitialisedVariableDuringTupleUnpacking`), this commit also adds the test cases `testUnionUnpackingIncludingListPackingSameItemTypes`, `testUnionUnpackingIncludingListPackingDifferentItemTypes`, and `testUnionUnpackingIncludingListPackingForVariousItemTypes`.