Functions

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