Add call/ret/push/pop instructions

* Call/ret/push/pop are implemented and appear to be working
* Call/ret/push/pop is specified in the 0x2000 block, replacing the
  mov instruction
* Mov instruction is now specified in the 0x3000 block

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

src/vm/mem.rs

@@ -90,6 +90,13 @@ impl<T> MemCursor<T>
                 Ok(Inst::$variant(source))
             }};
         }
+        macro_rules! dest {
+            ($variant:ident) => {{
+                let spec = (self.next_u8()? & 0xF0) >> 4;
+                let dest = self.next_dest(spec)?;
+                Ok(Inst::$variant(dest))
+            }};
+        }
         let inst = match op {
             ADD => dest_source!(Add),
             SUB => dest_source!(Sub),
@@ -110,6 +117,10 @@ impl<T> MemCursor<T>
             JMP => source!(Jmp),
             JZ => source!(Jz),
             JNZ => source!(Jnz),
+            CALL => source!(Call),
+            RET => Ok(Inst::Ret),
+            PUSH => source!(Push),
+            POP => dest!(Pop),
             MOV => dest_source!(Mov),
             HALT => Ok(Inst::Halt),
             NOP => Ok(Inst::Nop),