Refactoring of generator

[StefanosChaliasos]

We should apply a refactoring to the generator to use a table with possibilities instead of random.bool. Specifically, this table should include all supported features. We need that functionality for the following reasons.

1. For disabling features that a language cannot support in a fine-grained way.
2. For adding the ability for swarm testing.
3. It will be helpful for evaluation.
4. To minimise the effort of implementing a new language to (1) implementing a translator, (2) specifying this table for the new language, (3) specifying built-in types.