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 sometimes used in the printfk macro and in the rstd_print function to add a space
to the string argument.

Example:

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

    if mystr1 = "Hello world!" {
        printfk!(mystr1);
    } else {
        let output = add_str(mystr1, mystr2);
        printfk!(output);
    }

When add_str is called it checks if Rust std is being used. If std is being used it adds and returns the two strings, if
std isn't being used it prints the error message:

    add_str() isn't supported in no_std

This is because there isn't a way to add strings in fk_std without Rust std as of right now.