Trim spaces when deserialize numbers, booleans and characters

[Mingun]

Currently, deserialization of

<root>
  42
</root>

into usize, for example, is not possible, because number is surrounded by space characters. I was sure, that this was possible before #285, and still is possible, but realized:

• it was not possible, we are always returns error in such case
• Disable trim_text in Deserializer from_reader #285 introduced Text::trimmed, but it is not used... Probably because during polishing it was replaced by calls to Deserializer::skip_whitespaces, but it was intention, that the example above should work

So this PR adds missing tests and ensure that spaces around numbers and booleans are stripped. However, if after stripping text cannot be parsed as number or boolean, we call visit_str family of methods with the original, not trimmed string.

char case are more complicated, because we want to able to deserialize single space chars, but at the same time want to allow something like that:

<root>
  r
</root>

to be deserialized, as char r. To do that, if text content consist only from spaces, we pass that string to the visitor unchanged, otherwise spaces will be stripped and the trimmed string will be passed to the visitor.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.11%. Comparing base (38b44d4) to head (10d0646).
⚠️ Report is 65 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/mod.rs	75.00%	2 Missing ⚠️
src/de/simple_type.rs	90.90%	2 Missing ⚠️
src/utils.rs	83.33%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #912      +/-   ##
==========================================
+ Coverage   55.52%   58.11%   +2.58%     
==========================================
  Files          42       42              
  Lines       15511    15515       +4     
==========================================
+ Hits         8613     9016     +403     
+ Misses       6898     6499     -399     

Flag	Coverage Δ
unittests	58.11% <86.11%> (+2.58%)	⬆️

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