Big referencing refactor

- RC+RefCell pattern used... everywhere
- Ast type implemented
- unit tests for func_call
- more changes, but this commit scope has grown significantly and I
cannot list them all

src/vars.rs

@@ -15,22 +15,23 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-use std::boxed::Box;
+use std::cell::RefCell;
+use std::rc::Rc;
 use std::collections::HashMap;
-use crate::cell::{Ctr};
+use crate::segment::{Ctr};
 
-/* Mapping between a string token and a tree of Cells
+/* Mapping between a string token and a tree of Segments
  * The string token can be found in any Ctr::Symbol value
  * it is expected that the trees stored are already evaluated
  */
-pub type VTable = HashMap<String, Box<Ctr>>;
+pub type VTable = HashMap<String, Rc<Ctr>>;
 
 pub fn define(
-    vt: &mut Box<VTable>,
+    vt: Rc<RefCell<VTable>>,
     identifier: String,
-    var_tree: Box<Ctr>
+    var_tree: Rc<Ctr>
 ) {
-    if let Some(boxed_cell) = vt.insert(identifier, var_tree) {
-        drop(boxed_cell);
+    if let Some(rc_segment) = vt.borrow_mut().insert(identifier, var_tree) {
+        drop(rc_segment);
     }
 }