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