Rework health statistics exercise (#909)

* exercises: health-statistics: weight -> height

weight may be a sensitive topic for some readers; use height instead as this isn't important to the content of the course

* exercises: health-statistics: add health report

this lets us see a non-setter use case for &mut self

it also makes the 'statistics' side of this exercise more explicit as we count doctor visits

* exercises: health-statistics: normalize variable names

src/exercises/day-2/health-statistics.md

@@ -17,9 +17,9 @@ fn main() {
 }
 
 #[test]
-fn test_weight() {
+fn test_height() {
     let bob = User::new(String::from("Bob"), 32, 155.2);
-    assert_eq!(bob.weight(), 155.2);
+    assert_eq!(bob.height(), 155.2);
 }
 
 #[test]
@@ -29,4 +29,25 @@ fn test_set_age() {
     bob.set_age(33);
     assert_eq!(bob.age(), 33);
 }
+
+#[test]
+fn test_visit() {
+    let mut bob = User::new(String::from("Bob"), 32, 155.2);
+    assert_eq!(bob.doctor_visits(), 0);
+    let report = bob.visit_doctor(Measurements {
+        height: 156.1,
+        blood_pressure: (120, 80),
+    });
+    assert_eq!(report.patient_name, "Bob");
+    assert_eq!(report.visit_count, 1);
+    assert_eq!(report.blood_pressure_change, None);
+
+    let report = bob.visit_doctor(Measurements {
+        height: 156.1,
+        blood_pressure: (115, 76),
+    });
+
+    assert_eq!(report.visit_count, 2);
+    assert_eq!(report.blood_pressure_change, Some((-5, -4)));
+}
 ```