Use the external constant value MAGIC.

src/decoder.rs

@@ -5,10 +5,9 @@ use crate::{
 };
 
 pub fn try_read_magic<I: Iterator<Item=u8>>(bytes: &mut I) -> Result<(), DecodeError> {
-    let magic: [u8; 4] = [b'q', b'o', b'i', b'f'];
     for i in 0..4 {
         if let Some(letter) = bytes.next() {
-            if letter != magic[i] {
+            if letter != MAGIC[i] {
                 return Err(DecodeError::Magic)
             }
         } else {