Referenced objects and extendable object pool

[GwnDaan]

• Added member function to get all the objects by id, that have been referenced by the object in question
• Added ability to extend an ObjectPool by another binary encoded object pool

[codecov]

Codecov Report

Attention: 108 lines in your changes are missing coverage. Please review.

Comparison is base (bedbae3) 11.28% compared to head (799b88d) 10.87%.

Files	Patch %	Lines
src/object_pool/object.rs	0.00%	102 Missing ⚠️
src/object_pool/object_pool.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##           daan/fix-nullable-object-ids-      #41      +/-   ##
=================================================================
- Coverage                          11.28%   10.87%   -0.42%     
=================================================================
  Files                                 17       17              
  Lines                               2675     2778     +103     
=================================================================
  Hits                                 302      302              
- Misses                              2373     2476     +103     

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