Fix bug in function built-in with :documentation symbol

Previously it was only including the first expression of the function in the
closure. This because we took body from an element wise iterator. So instead we
have defined a new function `rest` that will take the remainder of a cons iterator.

src/core/cons/iter.rs

@@ -66,6 +66,11 @@ impl ElemIter<'_> {
         self.clone().fallible().count()
     }
 
+    /// Take the rest of the list as a cons.
+    pub(crate) fn rest(&self) -> Result<Option<&Cons>, ConsError> {
+        self.0.cons.transpose()
+    }
+
     pub(crate) fn fallible(self) -> fallible_iterator::Convert<Self> {
         fallible_iterator::convert(self)
     }

src/interpreter.rs

@@ -239,7 +239,11 @@ impl Interpreter<'_, '_> {
 
     /// Handle special case of (:documentation form) to build the docstring
     /// dynamically. If the docstring is not of this form, just return the current body.
-    fn replace_doc_symbol<'ob>(&mut self, quoted: &Rt<GcObj>, cx: &'ob mut Context) -> Result<GcObj<'ob>, EvalError> {
+    fn replace_doc_symbol<'ob>(
+        &mut self,
+        quoted: &Rt<GcObj>,
+        cx: &'ob mut Context,
+    ) -> Result<GcObj<'ob>, EvalError> {
         // quoted = ((<args..>) (doc_str) ...)
         let docstring = {
             let Ok(list) = quoted.bind(cx).as_list() else { return Ok(quoted.bind(cx)) };
@@ -263,10 +267,10 @@ impl Interpreter<'_, '_> {
             }
         };
         // ((<args..>) (:documentation <form>) body)
-        let mut forms = quoted.bind(cx).as_list().unwrap().fallible();
-        let arg_list = forms.next()?.unwrap();
-        let _old_doc = forms.next()?.unwrap();
-        let body = forms.next()?.unwrap_or_default();
+        let mut forms = quoted.bind(cx).as_list().unwrap();
+        let arg_list = forms.next().unwrap()?;
+        let _old_doc = forms.next().unwrap()?;
+        let body = forms.rest()?.map(|x| x.into()).unwrap_or(NIL);
         Ok(Cons::new(arg_list, Cons::new(docstring, body, cx), cx).into())
     }