Support byte-extracting from unions of non-fixed size

[tautschnig]

Various other types (including structs) were handled already, just unions were missing. Treat them much like a single-member struct.

The first commit is #5251, just the second commit is new.

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[smowton]

Suggest a test case for the corner no-elements / no-widest-element cases

[tautschnig]

Suggest a test case for the corner no-elements / no-widest-element cases

Done.