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())
-    }
-}

src/stl/control.rs

@@ -131,6 +131,7 @@ pub fn let_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
                             name: name.clone(),
                             args: Args::None,
                             conditional_branches: false,
+                            docs: "variable used in let form".to_string(),
                             value: ValueType::VarForm(Box::new(*var_val_res.unwrap().clone())),
                         },
                     );
@@ -166,7 +167,7 @@ pub fn let_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
         }
 
         if let Err(e) = res {
-            eprintln!("error evaluating let form: {}", e);
+            eprintln!("{}", e);
             return false;
         }