Functions

1. Rust main function
2. Rust hello world function
3. Rust function with parameter (&str)
4. Rust functions return the unit by default
5. Rust function return value (integer i32)
6. Return the last expression (no return)
7. Return a string
8. Rust recursive functions: factorial
9. Rust recursive functions: Fibonacci
10. Make function argument mutable inside the function
11. Cannot decalre the same function twice
12. Pass by reference to change external variable - Increment in a function
13. Count digits using functions
14. Function returning multiple values
15. Function accepting multiple types (e.g. any type of numbers)
16. Function that can accept any number (any integer or any float)
17. Exercise rectangle functions