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/Cargo.toml

@@ -5,4 +5,4 @@ edition = "2024"
 
 [dependencies]
 organelle = { path = "../organelle" }
-
+hyphae = { path = "../hyphae" }