♻️ internalize private methods of the circuit optimizer

[burgholzer]

Description

This PR internalizes all the private/protected methods in the circuit optimizer.
There is hardly any use in exposing them, so it's fine to just define them in the source file.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 89.81481% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 91.3%. Comparing base (868849c) to head (bcc48c1).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #585   +/-   ##
=====================================
  Coverage   91.3%   91.3%           
=====================================
  Files        132     131    -1     
  Lines      14052   14052           
  Branches    2243    2243           
=====================================
  Hits       12841   12841           
  Misses      1211    1211           

Flag	Coverage Δ
cpp	91.0% <89.8%> (ø)
python	99.5% <ø> (ø)

Files	Coverage Δ
src/CircuitOptimizer.cpp	88.4% <89.8%> (+<0.1%)	⬆️