Added implementation for ht_insert()
Added implementation for r_set_destroy()
Fixup: missing stdlib include
Merge pull request #161 from matthiasbeyer/implement-ht_insert

Added implementation for ht_insert()
Fix: The hash function should not alter the data
Merge pull request #196 from matthiasbeyer/fix-hash_function_const_parameter

Fix: The hash function should not alter the data
Merge pull request #183 from matthiasbeyer/implement-r_set_destroy

Added implementation for r_set_destroy()
Added implementation for r_set_remove() according to documentation in set.h header
Fixup: member accessing