Introduce various composite types

[Mesabloo]

The specification describes some composite types, namely:

• [x] 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)