Add object layout, object parsing, instruction layout

* Object layout and parsing are done in the vm::obj module
* Add MemCursor, a wrapper around the std::io::Cursor type for walking
  through VM memory
* Add vm::tick module for containing the Vm::tick() method
  implementation, since it's pretty big
* Instructions are now variable-sized, and are read lazily,
  one-at-a-time directly from memory.
* Add VM runtime error structure
* Probably some other stuff I forgot

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/reg.rs

@@ -5,7 +5,6 @@ macro_rules! registers {
         pub type Reg = u8;
 
         $(
-            #[allow(dead_code)]
             pub const $variant: Reg = $value;
         )*
     };
@@ -27,7 +26,9 @@ registers! {
     // Flags
     FLAGS = 3,
 
-    UNUSED00 = 4,
+    // Zero
+    NULL = 4,
+
     UNUSED01 = 5,
     UNUSED02 = 6,
     UNUSED03 = 7,