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>

hyphae/instructions.toml

@@ -53,7 +53,13 @@ description = "delete current stack frame"
 name = "load"
 args = ["src", "dest"]
 output = ""
-description = "copies src into dest"
+description = "shallow copies src into dest"
+
+[[instructions]]
+name = "dupl"
+args = ["src", "dest"]
+output = ""
+description = "deep copies src into dest"
 
 [[instructions]]
 name = "clear"
@@ -247,6 +253,12 @@ args = ["src"]
 output = ""
 description = "mutates a number datum into its inexact form"
 
+[[instructions]]
+name = "const"
+args = ["dst", "data"]
+output = ""
+description = "sets dst location to constant integer data"
+
 [[instructions]]
 name = "mkvec"
 args = []
@@ -300,4 +312,3 @@ name = "cdr"
 args = ["list"]
 output = "returns last element in cons cell"
 description = "takes an AST and returns last element in top level cons cell"
-