Support UNION structs in type analysis

[raehik]

Analysis.Types skips handling UNION structs. This means further processing doesn't have any information about that struct's structure, so certain statements can't be typed properly.

#238 found a runtime error that occurs when the type analysis finds an in-array assignment to a UNION struct: type information is missing, so it made a default decision which included an erroneous operation. The defaulting is now more careful, but we're still unable to type the statement properly without information on the union.