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.

tests/decode_images.rs

@@ -106,7 +106,7 @@ fn test_string_tags() {
         let path = PathBuf::from(TEST_IMAGE_DIR).join(filename);
         let img_file = File::open(path).expect("Cannot find test image!");
         let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
-        let software = decoder.get_tag(ifd::Tag::Software).unwrap();
+        let software = decoder.get_tag(tiff::tags::Tag::Software).unwrap();
         match software {
             ifd::Value::Ascii(s) => assert_eq!(
                 &s,

tests/encode_images.rs

@@ -1,9 +1,9 @@
 extern crate tiff;
 
 use tiff::decoder::{Decoder, DecodingResult};
-use tiff::decoder::ifd::Tag;
 use tiff::encoder::{colortype, TiffEncoder, SRational};
 use tiff::ColorType;
+use tiff::tags::Tag;
 
 use std::fs::File;
 use std::io::{Cursor, Seek, SeekFrom};