Change instruction encodings to avoid null instructions

Lots of null bytes in a row are a common thing, so the instructions
available can't start with a null pair of bytes anymore.

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

src/libvm/examples/deadbeef.asm

@@ -21,7 +21,7 @@
         jnz end
         mov %status, $1
     end:
-        div %r0, $0
+        int $0, $0
         halt
     .export main
 

src/libvm/src/inst.rs

@@ -23,32 +23,32 @@ macro_rules! instructions {
 pub type InstOp = u16;
 
 instructions! {
-    ADD     = 0x0000,
-    SUB     = 0x0001,
-    MUL     = 0x0002,
-    DIV     = 0x0003,
-    IDIV    = 0x0004,
-    MOD     = 0x0005,
-    AND     = 0x0006,
-    OR      = 0x0007,
-    XOR     = 0x0008,
-    SHL     = 0x0009,
-    SHR     = 0x000a,
-    INEG    = 0x000b,
-    INV     = 0x000c,
-    NOT     = 0x000d,
-    CMPEQ   = 0x1000,
-    CMPLT   = 0x1001,
-    JMP     = 0x1002,
-    JZ      = 0x1003,
-    JNZ     = 0x1004,
-    CALL    = 0x2000,
-    RET     = 0x2001,
-    PUSH    = 0x2002,
-    POP     = 0x2003,
-    INT     = 0x2004,
-    IRET    = 0x2005,
-    MOV     = 0x3000,
+    ADD     = 0x1000,
+    SUB     = 0x1001,
+    MUL     = 0x1002,
+    DIV     = 0x1003,
+    IDIV    = 0x1004,
+    MOD     = 0x1005,
+    AND     = 0x1006,
+    OR      = 0x1007,
+    XOR     = 0x1008,
+    SHL     = 0x1009,
+    SHR     = 0x100a,
+    INEG    = 0x100b,
+    INV     = 0x100c,
+    NOT     = 0x100d,
+    CMPEQ   = 0x2000,
+    CMPLT   = 0x2001,
+    JMP     = 0x2002,
+    JZ      = 0x2003,
+    JNZ     = 0x2004,
+    CALL    = 0x3000,
+    RET     = 0x3001,
+    PUSH    = 0x3002,
+    POP     = 0x3003,
+    INT     = 0x3004,
+    IRET    = 0x3005,
+    MOV     = 0x4000,
     HALT    = 0xF000,
     NOP     = 0xF001,
     DUMP    = 0xF002,

src/libvm/src/obj/assemble/session.rs

@@ -1,5 +1,4 @@
 use crate::{
-    addr::Addr,
     obj::{
         assemble::{
             Asm,

src/libvm/src/obj/disassemble.rs

@@ -1,4 +1,4 @@
-use crate::{mem::MemCursor, obj::obj::*, inst::Inst};
+use crate::{mem::MemCursor, obj::obj::*};
 use prettytable::{Table, row, cell};
 use std::io::{self, Write as Write};