Make reader and writer modules public, fix a small bug + polishing

[Mingun]

In anticipation of release 0.24, some polishing, plus fix of a minor bug in EndEventMismatch error generation.

Also, make reader and writer modules public, because the content of those modules are tend to grow and reexport of everything under a crate root seems to me a bad idea. All examples was updated to use the new recommended way to consume Readers and Writers.

Other commits contains some polishing.

I not yet finished checking documentation, so probably there will be one more PR before release, the last, I hope.

[codecov-commenter]

Codecov Report

Merging #456 (8644cea) into master (87d241a) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
+ Coverage   52.37%   52.38%   +0.01%     
==========================================
  Files          29       29              
  Lines       13562    13555       -7     
==========================================
- Hits         7103     7101       -2     
+ Misses       6459     6454       -5     

Flag	Coverage Δ
unittests	52.38% <66.66%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
benches/macrobenches.rs	0.00% <ø> (ø)
benches/microbenches.rs	0.00% <ø> (ø)
examples/custom_entities.rs	0.00% <ø> (ø)
examples/nested_readers.rs	0.00% <ø> (ø)
examples/read_buffered.rs	0.00% <ø> (ø)
examples/read_texts.rs	0.00% <0.00%> (ø)
src/de/mod.rs	82.55% <ø> (ø)
src/events/mod.rs	68.51% <ø> (ø)
src/lib.rs	12.83% <0.00%> (ø)
src/reader/async_tokio.rs	45.61% <ø> (ø)
... and 8 more

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