Utils for working with tags

Implement the encoder's `TiffValue` trait for the tags commonly defined
and represented as separate types in our own code. We don't tie the
tag for an entry of these to the value type (yet). This lets us keep the
trait exactly as-is and we avoid unnecessary churn. We can always add a
separate trait on top.

src/encoder/mod.rs

@@ -582,7 +582,7 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         )?;
         encoder.write_tag(Tag::XResolution, Rational { n: 1, d: 1 })?;
         encoder.write_tag(Tag::YResolution, Rational { n: 1, d: 1 })?;
-        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None.to_u16())?;
+        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None)?;
 
         Ok(ImageEncoder {
             encoder,