Implement EBML date element

[FreezyLemon]

This creates a newtype for EBML dates which is a thin wrapper around an i64 with some special handling for parsing and serialization. Technically, there is an optimization to be done here (see the FIXME), but that should be a pretty rare case.

[codecov-commenter]

Codecov Report

Patch coverage: 35.71% and project coverage change: -0.02 ⚠️

Comparison is base (537ddd4) 45.42% compared to head (72f60b9) 45.40%.

❗ Current head 72f60b9 differs from pull request most recent head c39743d. Consider uploading reports for the commit c39743d to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   45.42%   45.40%   -0.02%     
==========================================
  Files          14       14              
  Lines        2699     2711      +12     
==========================================
+ Hits         1226     1231       +5     
- Misses       1473     1480       +7     

Impacted Files	Coverage Δ
src/ebml/error.rs	48.14% <ø> (ø)
src/elements.rs	30.58% <0.00%> (ø)
src/serializer/ebml.rs	80.22% <0.00%> (-0.73%)	⬇️
src/serializer/elements.rs	13.12% <0.00%> (-0.06%)	⬇️
src/ebml/parse.rs	90.90% <83.33%> (-0.31%)	⬇️

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[lu-zero]

It is surely an improvement, when a 0-sized date is used btw?

[Luni-4]

@FreezyLemon

Is it possible to create a specific data item to manage zero-sized date?

Something like:

1. Parse field
2. If it is a zero-sized date, use ZeroSized data item with its own methods, otherwise use Date struct

I don't know if it's feasible though

[FreezyLemon]

@lu-zero A zero-sized Date Element will fall back to the default value, or the equivalent of 0 if no default exists (which means the date 2001-01-01T00:00:00.000000000 UTC).

But Matroska only has one Date Element in the spec currently. And that one has no default value. So at the moment, the only way to sensibly use a zero-sized Date Element in Matroska would be representing the exact date 2001-01-01 at midnight. I think that should be pretty rare.

@Luni-4 Sure, that sounds very possible. Should that be included in this PR?

[Luni-4]

@FreezyLemon we can add that in a later PR in my opinion

[Luni-4]

@FreezyLemon Can we open a new issue for the FIXME? In this way we don't forget about that, thanks!

[FreezyLemon]

@Luni-4 Sure, see #132