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

src/stl/append.rs

@@ -46,11 +46,3 @@ pub fn append_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
         Ok(Ctr::Seg(temp))
     }
 }
-
-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())
-    }
-}