- Create the enumerator `Antigen` that has 4 possibilities: A, B, O and AB And the enumerator `RhFactor` that has two possible values: Positive and Negative
- To provide a simple way to create blood types implement the trait FromStr for BloodType (which will allow us to use the `parse` method and the associated function from_str, so we can do:
```rust
let a_neg: BloodType = "A-".parse();
```
- Implement the std::cmp::Ord trait to make possible to sort a vector or array of BloodType's
- Implement the trait std::Debug for BloodType allowing to print a vector such as [BloodType { antigen: A, rh_factor: Positive}, BloodType{ antigen: B, rh_factor: Negative}] as [ A+, A-] using the formatting {:?}