Add tests for inline functions

examples/greeter.sb

@@ -2,6 +2,4 @@ fn main() {
     println(greet("World"))
 }
 
-fn greet(name: string): string {
-    return "Hello " + name
-}
+fn greet(name: string) = "Hello " + name

src/parser/tests.rs

@@ -37,6 +37,16 @@ fn test_parse_function_with_return() {
     assert!(tree.is_ok())
 }
 
+#[test]
+fn test_parse_inline_function() {
+    let raw = "
+    fn main() = 1 
+    ";
+    let tokens = tokenize(raw).unwrap();
+    let tree = parse(tokens, Some(raw.to_string()), "".into());
+    assert!(tree.is_ok())
+}
+
 #[test]
 fn test_parse_redundant_semicolon() {
     let raw = "