Added implementation for r_set_destroy()
Fixup: missing stdlib include
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 testing file for AVL implementation
Added allocation test for avl test
Added test for testing allocation and deleting for AVL
Added test for adding data to the avl tree
Added test for adding and destroying afterwards
Added test for adding multiple elements
Added test for adding multiple and destroying AVL afterwards
Added test for colliding inserts
Added test for finding a single element
Added test for finding multiple elements
[ci skip] Fixup: test_avl_add: The ptrs to the l/r next element should be NULL, actually
Fixed compiler errors about a const modified
Added a check to protect against segfaults
Fixes a bug in the avl_find test

I also updated the documentation of avl.h as
@neithernut forgot to update it.
Changed tests to conform to master