Add solution for sheet2

aufgaben/sheet2/sol3/count.rs

@@ -0,0 +1,19 @@
+//! Aufgabe 2.3
+
+fn main() {
+    let s = "The fat cat had a friend! ♥";
+    let count = count(s, 'a');
+    println!("{}", count);
+}
+
+
+fn count(haystack: &str, needle: char) -> u64 {
+    let mut count = 0;
+    for c in haystack.chars() {
+        if c == needle {
+            count += 1;
+        }
+    }
+
+    count
+}