memtx: allocate functional index key parts as tuples

Functional index keys are allocated and freed with MemtxAllocator's
alloc and free methods. In contrast to tuples, which are allocated and
freed with alloc_tuple and free_tuple, freeing a functional index key
happens immediately, irrespective of whether there's a snapshot in
progress or not. It's acceptable, because snapshot only uses primary
indexes, which can't be functional. However, to reuse the snapshot
infrastructure for creating general purpose user read views, we will
need to guarantee that functional index keys stay alive until all read
views using them are closed.

To achieve that, this commit turns functional index keys into tuples,
which automatically makes them linger if there's an open read view.
We use the same global tuple format for allocating functional keys,
because the key format is checked in key_list_iterator_next.

Closes #7376

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring