Added libreset/hash.h

This adds the libreset/hash.h file whichs purpose is to handle
everything related to the hash values (which are defined as uint64_t by
now).
Added hash member for the avl_el type
Added hash getter function for the avl_el type
Added libreset/element.h

This adds the libreset/element.h file, which contains code for handling
elements.
Replaced data member in avl_el with rs_element member
Added imports for hash.h and element.h headers
Only interact with the elements via the rs_element helper functions
Added implementation for find_element_with_parent() helper function