Misc fixes

[lee30sonia]

• Added [[nodiscard]] to the return value of cleanup methods (#526)
• Added missing inlines (#525)
• Include mockturtle.hpp in test.cpp such that all files will be included at least once in the tests. (However, it doesn't prevent people from forgetting to include new files in mockturtle.hpp!)

[codecov-commenter]

Codecov Report

Merging #528 (23ea412) into master (a3be48d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #528   +/-   ##
=======================================
  Coverage   82.61%   82.61%           
=======================================
  Files         146      146           
  Lines       18099    18099           
=======================================
  Hits        14953    14953           
  Misses       3146     3146           

Impacted Files	Coverage Δ
include/mockturtle/algorithms/cleanup.hpp	86.13% <100.00%> (ø)
include/mockturtle/generators/self_dualize.hpp	100.00% <100.00%> (ø)

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