ensure the m4 directory for autoreconf
simple gitignore to reduce the ouput of git-status
add iter_with_prefix for subtree iteration
extract repeated code as hattrie_iter_step
store remaining prefix in trie iter
filter out nodes in iter that fails the prefix check
clip prefix in key retrieving
refine logic of hattrie_iter_prefix_not_match, fix negative level
add a fast control path for short or none prefix searches