Finish garbage collected heap

This commit adds unit tests for garbage collection as well as a
deep copy implementation for cons which fully deep copies the whole
tree and a clone implementation for Datum which deep copies cons.

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

hyphae/src/vm.rs

@@ -102,7 +102,7 @@ impl VM {
                     Address::Oper3 => &self.oper[2],
                     Address::Oper4 => &self.oper[3],
                     Address::Stack => &self.stack[$oper.1],
-                    Address::Numer => e!("cannot access constant numeric"),
+                    Address::Numer => e!("cannot access constant numeric data"),
                     Address::Instr => e!("bad access to instruction data"),
                 }
             };