Support 64-bit encoding

Note that this adds the LONG8 data type from BigTIFF because of bounds
requiring TiffValue. However, this datatype should not actually be
written to tags.

src/decoder/ifd.rs

@@ -122,6 +122,8 @@ pub enum Type(u16) {
     SSHORT = 8,
     SLONG = 9,
     SRATIONAL = 10,
+    /// BigTIFF 64-bit unsigned integer
+    LONG8 = 16,
 }
 }