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/eval.rs

@@ -39,10 +39,7 @@ pub fn eval(ast: &Seg, syms: &mut SymTable) -> Result<Box<Ctr>, String> {
     let mut none = false;
     while !none {
         match &**arg_car {
-            Ctr::Seg(ref inner) => match eval(inner, syms) {
-                Ok(res) => car = res,
-                Err(e) => return Err(format!("evaluation error: {}", e)),
-            },
+            Ctr::Seg(ref inner) => car = eval(inner, syms)?,
 
             Ctr::Symbol(ref tok) => {
                 let outer_scope_seg_holder: Seg;