Support array access

[niknetniko]

Add support for a new "function type": the array access.

While this adds support in the DSL and in the code generation, I would still consider it experimental on the whole, since:

• We have no uniform behaviour for out of bounds indices (e.g. most languages will throw, but each with their own error)
• We have no uniform behaviour for negative indices (e.g. Python supports it, but most other languages don't)
• We have ugly support in Java for Lists and Arrays: as we don't really have a "type" system, we cannot know if the type of a variable is an array or a list. We thus need to call a wrapper function get which determines it at runtime. Array access in Java thus looks like get(arrayOrList, index) instead of array[index] and list.get(index).

Fixes #325.

Since we will use the [] operator for arrays, we'll use a "magic" function for dictionary/map/object access. That is even more inconsistent in behaviour, so that well be done in another PR eventually.

[niknetniko]

Closing for now, as this isn't a good enough solution.