Factor tags into own module

Tag representations are not decoder- or encoder-specific, so should live
in their own module accessible to both for reuse. The decoder::ifd
module remains since this module is mainly dedicated to decoding the
IFD.

src/lib.rs

@@ -13,6 +13,7 @@ extern crate miniz_oxide;
 pub mod decoder;
 pub mod encoder;
 mod error;
+pub mod tags;
 
 pub use self::error::{InflateError, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError};