add a way to import other `.nat` files in the REPL and other files

[hydrolarus]

This would allow more re-use of definitions.

Ideally for disambiguation also a namespacing mechansim should be present.

Maybe for "import everything" and "only import the name of the namespace" different statement types could be used?

• import OtherFile for loading OtherFile.nat from a set of known locations and then making all definitions usable
• use OtherFile for loading OtherFile.nat from a set of known locations and then allowing the use of definitions using OtherFile.some_name