Prefer binary realloc to self-managed buffers

Use erlang's allocator and simplify the code by writing into the
buffer we're going to give back anyway.

Also introduce a function to NUL-terminate iolist buffers, which can
avoid memory copying.

c_src/commit.c

@@ -40,7 +40,6 @@ geef_commit_tree(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[])
 	term_obj = enif_make_resource(env, obj);
 	enif_release_resource(obj);
 
-
 	return enif_make_tuple3(env, atoms.ok, atoms.tree, term_obj);
 
 }