Warn when non-void methods and closures finish without returning

tests/fibonacci_sequence.aer

@@ -13,7 +13,7 @@ class Program {
 		return $cur;
 	}
 
-	public int main() {
+	public void main() {
 		var_dump($this->fib(43));
 	}
 }

tests/singleton_test.aer

@@ -22,7 +22,7 @@ final class Test {
 		return $this->value;
 	}
 
-	public int set(int $value = 0) {
+	public void set(int $value = 0) {
 		$this->value = $value;
 	}
 }