Implement read_text - a method that returns a text between two tags

[Mingun]

This method introduces Reader::read_text and NsReader::read_text methods, that returns text between open and closed tags.

At the same time it removes Reader::read_text_into, because:

• it is have the name similar to the newly added method, but behaves totally different
• it is trivially to implement it manually
• as was noted in skips events on failed read_text #154, the behavior of this method should be changed to match the behavior of the new method, but this is hard to implement currently, because when we read events, we store in the user-provided buffer only data, that required for event content, but omitting the part corresponding to markup. But new method requires that bytes too. Implementing this behavior requires further investigations, therefore, I have decided so far to make an implementation only for the borrowing reader.

[codecov-commenter]

Codecov Report

Merging #455 (792d23d) into master (2bf2d2d) will increase coverage by 0.03%.
The diff coverage is 19.14%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage   52.35%   52.38%   +0.03%     
==========================================
  Files          29       29              
  Lines       13565    13562       -3     
==========================================
+ Hits         7102     7105       +3     
+ Misses       6463     6457       -6     

Flag	Coverage Δ
unittests	52.38% <19.14%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
examples/read_texts.rs	0.00% <0.00%> (ø)
src/events/attributes.rs	93.90% <0.00%> (-0.20%)	⬇️
src/reader/async_tokio.rs	45.61% <0.00%> (ø)
src/reader/buffered_reader.rs	86.24% <0.00%> (+4.83%)	⬆️
src/reader/mod.rs	90.38% <ø> (+0.13%)	⬆️
src/reader/ns_reader.rs	63.39% <0.00%> (-1.75%)	⬇️
src/reader/slice_reader.rs	94.82% <37.50%> (-5.18%)	⬇️
src/de/mod.rs	82.55% <50.00%> (-0.02%)	⬇️
src/events/mod.rs	68.51% <80.00%> (-0.09%)	⬇️
... and 6 more

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