Add various refactorings

[tranzystorekk]

Highlights:

• Remove or simplify redundant code, like &*, .as_ref(), etc.
• Use IntoIterator to avoid some invocations like .iter()
• Simplify some Option-oriented code

[codecov-commenter]

Codecov Report

Merging #341 (8635188) into main (bc5b6fd) will increase coverage by 0.22%.
The diff coverage is 43.15%.

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
+ Coverage   70.13%   70.35%   +0.22%     
==========================================
  Files          46       46              
  Lines        6756     6737      -19     
==========================================
+ Hits         4738     4740       +2     
+ Misses       2018     1997      -21     

[yihuaf]

This is great! Thanks for doing this.