Handle deprecated Matroska Elements

[FreezyLemon]

Closes #117.

There was a bug in skip_master (now skip_element) where it didn't skip all of the Element data which was easy to fix.

Also removed some serialization functions that aren't used anymore.

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: +1.64 🎉

Comparison is base (8944849) 46.84% compared to head (703875e) 48.49%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
+ Coverage   46.84%   48.49%   +1.64%     
==========================================
  Files          14       14              
  Lines        2741     2584     -157     
==========================================
- Hits         1284     1253      -31     
+ Misses       1457     1331     -126     

Impacted Files	Coverage Δ
src/ebml/mod.rs	25.00% <ø> (ø)
src/elements.rs	33.48% <ø> (+2.46%)	⬆️
src/muxer.rs	0.00% <ø> (ø)
src/serializer/cookie_utils.rs	59.75% <ø> (+2.10%)	⬆️
src/serializer/ebml.rs	86.23% <ø> (+6.23%)	⬆️
src/serializer/elements.rs	14.65% <0.00%> (+1.52%)	⬆️
src/permutation.rs	86.48% <80.00%> (-2.09%)	⬇️
src/demuxer.rs	88.18% <100.00%> (-0.74%)	⬇️
src/ebml/parse.rs	90.90% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Luni-4]

Thanks a lot! This is really fantastic!

Just a simple nit