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
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()