added size functions for linked list and hash map
added keys function for hash map
refactoring test_keys
stddef
Merge pull request #9 from seanfisk/size_keys

Size keys
Merge branch 'master' into remove-from-hash-map

Conflicts:
	src/hash_map.c
	src/hash_map.h
	test/test_hash_map.c
	test/test_linked_list.c