lua: add non-recursive msgpack decoding functions

Needed for #3276.

@TarantoolBot document
Title: Non-recursive msgpack decoding functions

Contracts:

```
msgpack.decode_array_header(buf.rpos, buf:size()) -> arr_len, new_rpos
msgpack.decode_map_header(buf.rpos, buf:size()) -> map_len, new_rpos
```

These functions are intended to be used with a msgpack buffer received
from net.box. A user may want to skip {[IPROTO_DATA_KEY] = ...} wrapper
and an array header before pass the buffer to decode in some C function.

See https://github.com/tarantool/tarantool/issues/2195 for more
information re this net.box's API.