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

[haraldmaida]

Borrowed &Result can not be asserted. Assertions like

assert_that!(&Ok(vec![1, 2, 3])).is_ok();
assert_that!(&Err("severe failure".to_string())).is_err();
assert_that!(&Ok(vec![1, ,2, 3])).has_value(&vec![1, 2, 3]);
assert_that!(&Ok(vec![1, 2, 3])).ok().contains_exactly(&[1, ,2 ,3]);
assert_that!(&Err("severe failure".to_string())).has_error_message("severe failure");

are expected to work, but result in compile errors.

Implemented assertions for Result for borrowed &Result as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.31%. Comparing base (60049e0) to head (22866af).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   96.18%   96.31%   +0.12%     
==========================================
  Files          26       26              
  Lines        2965     3065     +100     
  Branches     2965     3065     +100     
==========================================
+ Hits         2852     2952     +100     
  Misses        109      109              
  Partials        4        4              

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