Add more values to the ast to facilitate encoding

- Add a instruction value that contains the encoding, the address and a
  flag to indicate if this instruction contains label references
- Add label value that contains an address
- Add reference value that contains offset, an absolute address and an
  operand size
- define types for all value options in the union
- define accessor functions for all the values in the union

src/main.c

@@ -88,7 +88,8 @@ error_t *print_encoding(tokenlist_t *list) {
         if (node->id != NODE_INSTRUCTION)
             continue;
 
-        print_hex(node->value.encoding.len, node->value.encoding.encoding);
+        print_hex(node->value.instruction.encoding.len,
+                  node->value.instruction.encoding.buffer);
     }
 
     encoder_free(encoder);