Fixed the GetItemExpression

[stof]

It is currently totally unusable: adding it in the expression throws a fatal error because of the missing interface, and its compilation is totally broken.

[schmittjoh]

I'm not sure whether we should not completely remove this feature.

I've never used it myself, and it seems a bit too much for expressions; after all, you shouldn't start to program in there. Could you maybe share your use-case?

[stof]

The only reason why I use it currently is because I'm doing a weird hack for now to be able to workaround the missing visitors rewriting the AST.

But it should either be fixed or removed. Keeping it totally broken does not make sense.

On the other hand, if your application uses arrays to store the values instead of using objects with getters and setters, it may make sense to allow accessing items. However, should the item key really be an expression in the parser ? Allowing only strings or integers may be better.