add_str is a function that adds two strings and returns them as one. This can be useful for things like adding missing
words/characters to input, add_str is used in the printfk macro to add a space to the string argument which shouldn't
have a space.

Example:

    let mystr1 = "Hello ";
    let mystr2 = "world!";

    if mystr1 = "Hello world!" {
        printfk("mystr1 = Hello world!!");
    } else {
        add_str(mystr1, mystr2);
        // Adds mystr2 to mystr1
        printfk("mystr1 = Hello world!!");
    }
