fix: assertions for Option are not available for borrowed &Option

[haraldmaida]

Currently a borrowed Option can not be asserted. Assertions like:

assert_that!(&Some(42)).is_some();
assert_that!(&Some([1, 2, 3])).some().contains(&2);

are expected to work but result in compile errors.

Implemented assertions for Option for borrowed &Option as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.18%. Comparing base (6a21d33) to head (5018bae).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   96.10%   96.18%   +0.08%     
==========================================
  Files          26       26              
  Lines        2902     2965      +63     
  Branches     2902     2965      +63     
==========================================
+ Hits         2789     2852      +63     
  Misses        109      109              
  Partials        4        4              

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