Introduce various composite types

[Mesabloo]

The specification describes some composite types, namely:

• structure types (t1, t2, ..., tn)
• union types <t1, t2, ..., tn>

Structure types are basically what we call tuples in higher-level programming languages like Haskell, whereas union types are the same thing (but un-named) as in C.
Note that, because of alignment/padding and various other reasons, the size of the structure is not exactly the sum of the size of all fields. (This would be true if structures presented here were not packed)