Rust Common Collections - Exercise 1

In the book The Rust Programming Language by Steve Klabnik and Carol Nichols, chapter 8 - Common Collections - presents 3 additional exercises. This is my solution to exercise 1:

• Given a list of integers, use a vector and return the mean (the average value), median (when sorted, the value in the middle position), and mode (the value that occurs the most often; a hash map will be helpful here) of the list.