Fixed bug with VM assigning wrong opcode

- Fixed a bug with VM assigning wrong opcode. After doing a little digging into the Iridium 1 source code (skipping to the next part of the tutorial), I discovered I had to have implement from() function in both directions. I.e. the Opcode outputs integer and integer outputs an Opcode. Derp moment in retrospective since I already had it done for the former but this is a learning experience.
- Moved Opcode enum to assembler module.

src/assembler/instruction_parser.rs

@@ -21,7 +21,7 @@ impl AssemblerInstruction {
         match self.opcode.to_owned() {
             Token::Opcode { code } => match code {
                 _ => {
-                    results.push(code as u8);
+                    results.push(code.into());
                 }
             },
             _ => {
@@ -30,6 +30,7 @@ impl AssemblerInstruction {
             }
         };
 
+
         for operand in vec![&self.op1, &self.op2, &self.op3] {
             match operand {
                 Some(t) => AssemblerInstruction::extract_operand(t, &mut results),
@@ -81,7 +82,7 @@ named!(pub instruction_one<CompleteStr, AssemblerInstruction>,
 #[cfg(test)]
 mod instruction_parser_test {
     use super::*;
-    use crate::instruction::Opcode;
+    use crate::assembler::Opcode;
 
     #[test]
     fn test_parse_instruction_form_one() {