Add: avl function attributes
Add: nonnull macros for avl/common.h
Merge pull request #251 from matthiasbeyer/add-avl_attributes

Add attributes to avl functions
Add implementation for r_set_equal()
Added ll_equal() implementation
Started implementation for avl_equal()
Started implementation for ht_equal()
Added missing entries in CMakeLists file
Removed unneeded variable
Removed unneeded avl_equal function
Added implementation for ht_equal
Implemented avl_is_subset
Implemented ll_is_subset
Added source files to CMakeLists.txt
Fixed pointer type
Added missing file to CMakeLists
Added declaration of ll_is_subset
Fixed types
Added ll_is_subset tests
Fixed return values