add str::substring

[lovasoa]

@ISibboI , the usize overflow was already handled, but I changed the behavior to raise an error instead of returning an empty string when supplied indices are out of bounds. Let me know if that's ok :)

[ISibboI]

Also, in case I get to implement #159 before merging this, it may be good to use it as well. But I can also do that.

[ISibboI]

Yeah, now you can use wrong_function_argument_amount_range instead of wrong_function_argument_amount to report that the new function can handle two or three arguments.

[lovasoa]

Okay, I'm leaving this in your hands. thanks !

[lovasoa]

Thank you 🙏