Several changes, see commit msg

* clean up all tests
* bugfix for zero value functions, and test
* removed expand function, put in snippets
* added doc strings to Symbol type
* added doc strings to symbol declarations
* implemented display for Args type
* wrote a help function
* wrote docstrings for all builtins and config vars

snippets/curiosity_expand_callback.rs

@@ -0,0 +1,7 @@
+pub fn expand_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(_) = *ast.car {
+        Ok(*ast.car.clone())
+    } else {
+        Err("non list passed to expand!".to_string())
+    }
+}