Marked new_avl_el() as malloc function
Add newlines to bloom subsystem for possible future attribute usage
Added attributes for avl functions
Added nonnull attributes for ht functions
Add attribute to isolate_root_node()
Moved: attributes.h to public interface
Fix: Include guard in attributes.h
Added note in attributes.h on why the header is public
Add nonnull attributes to set header
Merge pull request #254 from matthiasbeyer/add-function_attributes

[WIP] Add function attributes
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