Encode values immediately

This currently avoids buffering the collection, but not buffering them
individually where we still format it into a byte vector.

The main drive here is making the DirectoryEncoder less dependent on the
values. That is, it should be concerned with the Entry representation of
values only. A few decoders expect all values to strictly follow the
image directory and the data layout of such files should be controllable
by the user. This could be achieved by encoding dummy entries which are
later corrected once the actual item is known by seeking back and
writing the directory anew.

By no longer assuming values to be available on finishing a directory we
also prepare an interface that encodes an existing directory.

tests/encode_images.rs

@@ -57,7 +57,7 @@ fn encode_decode() {
                 ),
                 (Tag::Compression, ifd::Value::Short(1)),
                 (Tag::PhotometricInterpretation, ifd::Value::Short(2)),
-                (Tag::StripOffsets, ifd::Value::Unsigned(8)),
+                (Tag::StripOffsets, ifd::Value::Unsigned(47)),
                 (Tag::SamplesPerPixel, ifd::Value::Short(3)),
                 (Tag::RowsPerStrip, ifd::Value::Unsigned(3334)),
                 (Tag::StripByteCounts, ifd::Value::Unsigned(30000)),
@@ -137,7 +137,7 @@ fn encode_decode_big() {
                 ),
                 (Tag::Compression, ifd::Value::Short(1)),
                 (Tag::PhotometricInterpretation, ifd::Value::Short(2)),
-                (Tag::StripOffsets, ifd::Value::UnsignedBig(16)),
+                (Tag::StripOffsets, ifd::Value::UnsignedBig(27)),
                 (Tag::SamplesPerPixel, ifd::Value::Short(3)),
                 (Tag::RowsPerStrip, ifd::Value::Unsigned(3334)),
                 (Tag::StripByteCounts, ifd::Value::UnsignedBig(30000)),