feat!: implement Not expectation combinator and revise failure messages

[haraldmaida]

Having a "not" combinator for expectations makes writing (custom) assertions more convinient.

Implemented the Not expectation combinator and adapted all implementations of currently provided expectations to make use of the Not combinator if feasible.

Additionally revised all failure messages mainly changing the phrases "is" or "has" to "to be" or "to have".

BREAKING CHANGES:

• the signature of the trait method Expectation::message() is changed. The method message() has got the additional argument inverted: bool which is set when an expectation has been inverted with the Not combinator expectation.
• all failure messages of assertions have been revised to use the phrases "to be..." and "to have..." instead of "is..." and "has..."

[codecov]

Codecov Report

Attention: Patch coverage is 93.57602% with 60 lines in your changes missing coverage. Please review.

Project coverage is 95.71%. Comparing base (0d1a1a7) to head (eedf13a).

Files with missing lines	Patch %	Lines
src/string/mod.rs	87.30%	4 Missing and 12 partials ⚠️
src/char/mod.rs	94.66%	2 Missing and 6 partials ⚠️
src/number.rs	93.69%	3 Missing and 4 partials ⚠️
src/order/mod.rs	89.55%	1 Missing and 6 partials ⚠️
src/char_count.rs	88.88%	0 Missing and 6 partials ⚠️
src/length.rs	90.90%	0 Missing and 6 partials ⚠️
src/range/mod.rs	89.13%	5 Missing ⚠️
src/result/mod.rs	90.90%	0 Missing and 2 partials ⚠️
src/error/mod.rs	95.65%	0 Missing and 1 partial ⚠️
src/iterator/mod.rs	98.87%	0 Missing and 1 partial ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   96.70%   95.71%   -0.99%     
==========================================
  Files          31       31              
  Lines        3425     3899     +474     
  Branches     3425     3899     +474     
==========================================
+ Hits         3312     3732     +420     
- Misses        109      118       +9     
- Partials        4       49      +45     

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