key to look up mapping for
true: verify that the mapping exists, false: the mapping may or may not exist
a pointer to the element mapped to by the specified key or null if not found and must_exist is false. The caller should make sure that the key is not changed.
Out: - The returned array can only be null if must_exist is false. - The length of the returned array is V unless the array is null.
Looks up a mapping from the specified key.