Support for index-based arrays slicing

[flexferrum]

Jinja2 index operator [] follows the python arrays slicing semantic: https://stackoverflow.com/questions/509211/understanding-slice-notation

It needs to be supported.

[layderv]

For the parser, it looks like ParseSubscript() is to be updated to support [ : : ].

For the evaluator, I think SubscriptExpression::Evaluate is the one to edit, but I don't quite understand how / the types that are involved in that method. Can I get any suggestion please?

[flexferrum]

For the evaluator

I've considered to add new AST node for this purpose, something like SliceExpression, because indexing and slicing has got too different behavior.