Replace print/println keywords with generic function call mechanism

- Add NODE_CALL with name, args, and arg_count to parser
- Add TOK_COMMA token and tokenize (, ), , in lexer
- Remove TOK_PRINT/TOK_PRINTLN keywords; print/println are now regular
  identifiers resolved as built-in functions in the evaluator
- Add NODE_CALL debug output in ast_print

projects/functions.j

@@ -0,0 +1,11 @@
+x = "Hello world!"
+y = 21
+println()
+println(x)
+print("y=")
+println(y)
+
+dummy()
+
+if y > 1:
+    println("OK!")