Mutex is a container data structure commonly held by smart pointers that takes in data and lets us borrow mutable
and immutable references to the data within. This prevents borrowing from being abused by
having the operating system restrict only one CPU thread at time to have access to the data,
blocking other threads until that original thread is done with its locked borrow.
Multithreading is beyond the scope of Tour of Rust, but Mutex is a fundamental part of orchestrating
multiple CPU threads accessing the same data.
There is a special smart pointer Arc which is identical to Rc except uses thread-safe incrementing of reference counts.
It's often used to have many references to the same Mutex.