feat: custom errors

[rymnc]

Cleaner to read the RLN contract now, better to test too

[github-actions[bot]]

LCOV of commit ab78e5d during CI #12

Summary coverage rate:
  lines......: 100.0% (497 of 497 lines)
  functions..: 100.0% (9 of 9 functions)
  branches...: 100.0% (22 of 22 branches)

Files changed coverage rate:
                              |Lines       |Functions  |Branches    
  Filename                    |Rate     Num|Rate    Num|Rate     Num
  ==================================================================
  contracts/Rln.sol           | 100%     36| 100%     7| 100%     22