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>

organelle/src/lib.rs

@@ -73,6 +73,12 @@ pub enum Number {
     Sym(SymbolicNumber)
 }
 
+impl Default for Number {
+    fn default() -> Self {
+        Number::Fra(Fraction(0, 1))
+    }
+}
+
 impl From<SymbolicNumber> for Number {
     fn from(value: SymbolicNumber) -> Self {
         Number::Sym(value)