feat(c3) : Add ability to evaluate cost of C3 plan

[ebianchi]

Addresses #31

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This change is 

[github-actions]

LCOV of commit 1daabf9 during C3 Coverage #90

Summary coverage rate:
  lines......: 89.4% (1653 of 1850 lines)
  functions..: 77.1% (135 of 175 functions)
  branches...: 50.0% (1278 of 2554 branches)

Files changed coverage rate:
                                            |Lines       |Functions  |Branches    
  Filename                                  |Rate     Num|Rate    Num|Rate     Num
  ================================================================================
  core/c3.cc                                | 8.6%    279|2733%    21|    -      0
  core/c3.h                                 |14.3%     28|1800%     3|    -      0

⛔ The code coverage is too low: 89.35. Expected at least 90.