Add possibility of forward declarations

[lukaszcz]

This has come up during Advent of Code.

Example:

• you have a large constant data which you use in the file,
• you want to put it at the end because it's too big, but it's not currently possible - variables need to be defined before being referenced,
• a workaround is to define data in a separate module and import the module.

One could want to have it in the same module, but at the end, with a forward declaration at the beginning.

[janmasrovira]

I believe we explicitly decided to have variables (or definitions that are not function-like) to not be recursive, and hence, not 'forward-referencable'. If having data at the end of the file is the only use case we have for this, I don't think it justifies any change in the language.
Moreover, even if it was possible, I think it'd be better style to have data-like definitions in a separate file.