Improve collection library usage in snippet compiler

[abelhegedus]

To support the built-in SET type, #376 changed the usage of collection variable descriptors from specific Type to String identifier. This is needed as the built-in collection types have no corresponding xUML-RT pairs. However, this should be improved in the future to be able to use Types again in all cases.

[abelhegedus]

For future reference: look at CollectionLiteralBuilder and OperationCallBuilder.
However, it seems to be that at least the later is never used now that CollectionOperationCallBuilder exists, while the former should probably be rewritten with a similar approach.