Remove requirement to have Reader when decoding attributes and write anything converted to Event

[Mingun]

This PR includes two changes, which both related to my work of splitting event into two event classes -- for reading and for writing. Events for reading then would only borrow data and this will help to fix #332 and also open doors to no_std reader (not sure why it would be needed although).

The first commit just removes unused import that I noticed while working on this.

The second commit relaxes requirements for decoding values from Attribute. If you created attribute for writing and then for whatever reason wants to read its value back, you need a Reader which you does not have. This commit fixes that.

The third commit makes writing methods take Into<Event> instead of AsRef<Event>. The former trait give a lesser abilities then Into. It allows you to pass your own struct to write, but you need to have event in your struct to be able to take a ref to it. Into would allow you to construct event on demand and also would allow to directly pass reader event to the writer method that would require writer event if (when?) event splitting will be implemented. In any case, I think, that taking Into is better.

If there are any objections, then unnecessary commits can be removed from PR.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.18182% with 14 lines in your changes missing coverage. Please review.

Project coverage is 60.30%. Comparing base (7558577) to head (55f7aa1).
Report is 43 commits behind head on master.

Files	Patch %	Lines
src/events/attributes.rs	0.00%	6 Missing ⚠️
benches/macrobenches.rs	0.00%	4 Missing ⚠️
examples/read_nodes.rs	0.00%	3 Missing ⚠️
examples/custom_entities.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
- Coverage   61.81%   60.30%   -1.52%     
==========================================
  Files          41       41              
  Lines       16798    16191     -607     
==========================================
- Hits        10384     9764     -620     
- Misses       6414     6427      +13     

Flag	Coverage Δ
unittests	60.30% <68.18%> (-1.52%)	⬇️

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[Mingun]

Usability of the last commit can be demonstrated in #766. There I plan to introduce conception of low-level RawReader which produces RawEvents. Those are current Reader and Event types. Also I plan introduce conception of high-level Reader which would produce new Events.

I plan to make RawEvent borrow-only, i.e. they will contain &[u8], and Event will contain Cow<[u8]>, because new Reader would produce events from different files. However, I'd want to be able to write both types of events using the same Writer (there are no reasons to have RawWriter). Because Event will not contain RawEvent inside, it will not be possible to implement AsRef<RawEvent> for it.

[dralley]

However, I'd want to be able to write both types of events using the same Writer (there are no reasons to have RawWriter). Because Event will not contain RawEvent inside, it will not be possible to implement AsRef for it.

@Mingun is it possible to go the opposite direction, i.e. have RawEvent implement Into<Event>?

[Mingun]

Yes, it will be done that way.