Fix unordered_set reserve

[benedictkao]

• Reserve was broken because of 2 reasons:
  • Undefined behaviour/memory leak when new_size was smaller than current _bucket_count
  • Did not rehash existing elements, which could cause duplicate elements to be put into the set
• Also added:
  • max_load_factor (https://cplusplus.com/reference/unordered_set/unordered_set/max_load_factor/)
  • bucket_count (https://cplusplus.com/reference/unordered_set/unordered_set/bucket_count/)
• TODO:
  • Move implementation into .tpp file
  • Remove use of std::ceil (https://stackoverflow.com/questions/8377412/ceil-function-how-can-we-implement-it-ourselves)

[chris-nee]

Lgtm

Wondering if we can optimise the rehashing process

Came across this some time back - https://en.wikipedia.org/wiki/Consistent_hashing

Will add it in issues for future enhancements