refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/hashtab.c

@@ -87,7 +87,7 @@ void hash_clear_all(hashtab_T *ht, unsigned int off)
 ///                  is changed in any way.
 hashitem_T *hash_find(const hashtab_T *const ht, const char *const key)
 {
-  return hash_lookup(ht, key, STRLEN(key), hash_hash((char_u *)key));
+  return hash_lookup(ht, key, strlen(key), hash_hash((char_u *)key));
 }
 
 /// Like hash_find, but key is not NUL-terminated