Allow function calls in expressions and add len() built-in

- Parse function calls in parse_term() so they work inside expressions
  (e.g. z = len(x), y = len(x) + 1)
- Add len() built-in for string length in evaluator

projects/functions.j

@@ -1,11 +1,2 @@
-x = "Hello world!"
-y = 21
-println()
-println(x)
-print("y=")
-println(y)
-
-dummy()
-
-if y > 1:
-    println("OK!")
+x = "Hello"
+println(len(x) + 3)