Add Disassemble, fix bug in position calculation

* Disassemble structure can be used for dumping an object section
* Assembler position calculation was messing up, causing jump addresses
  to be wrong. This is fixed.

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

src/vm/reg.rs

@@ -7,6 +7,15 @@ macro_rules! registers {
         $(
             pub const $variant: Reg = $value;
         )*
+
+        pub fn reg_name(reg: Reg) -> Option<&'static str> {
+            match reg {
+                $(
+                    $value => Some(stringify!($variant)),
+                )*
+                _ => None,
+            }
+        }
     };
 }
 
@@ -91,5 +100,6 @@ registers! {
     R47 = 61,
     R48 = 62,
     R49 = 63,
-    LAST_REG = R49,
 }
+
+pub const LAST_REG: Reg = R49;