HyphaeVM Garbage Collection

This commit removes the dependency upon Mycelium::sexpr::Datum from Hyphae
and instead adds a heap.rs module including three types:

- Gc: This is essentially a Box<Rc<T>> but passed around as a *const Rc<T>.
    not only does this allow the wrapped T to be passed around in a format
    that fits completely within a single physical register, this also
    applies a greedy reference counting garbage collection to each and
    every object allocated in the VM.

- Datum: This is a simplified enum for type polymorphism. Similar to the
    original Mycelium::sexpr::Datum.

- Cons: This is a very standard Cons cell type.

Additionally, two new instructions are added:
- DUPL: a deep copy instruction
- CONST: an instruction to create number datum from embedded constants
- NTOI: casts a number to its inexact form
- NTOE: casts a number to its exact form

Fixes: #35 and #36

Signed-off-by: Ava Affine <ava@sunnypup.io>

mycelium/src/sexpr.rs

@@ -110,34 +110,6 @@ impl fmt::Debug for Datum {
 pub struct Ast(pub Rc<Datum>, pub Rc<Datum>);
 
 impl Ast {
-    pub fn subsl(&self, start: isize, end: isize) -> Ast {
-        if end - start == 1 {
-            return Ast(Rc::from(self[start as usize].clone()), Rc::from(Datum::None))
-        }
-
-        if end == 0 {
-            return Ast(
-                Rc::from((*(self.0)).clone()),
-                Rc::from(Datum::None)
-            )
-        }
-
-        let Datum::List(ref next) = *self.1 else {
-            panic!("index into improper list form")
-        };
-
-        if start <= 0 {
-            Ast(
-                Rc::from((*(self.0)).clone()),
-                Rc::from(Datum::List(
-                    Rc::from(next.subsl(start - 1, end - 1))))
-            )
-
-        } else {
-            next.subsl(start - 1, end - 1)
-        }
-    }
-
     pub fn len(&self) -> usize {
         let Datum::List(ref next) = *self.1 else {
             return 1