The patch introduces C and Lua methods for extracting position
of tuple in index. Multikey and functional indexes are not supported.

Also this patch extends method index_create_iterator_after and adds
implementation of iterator_position for memtx tree. Options
after and fetch_pos are added to Lua select as well (both FFI and LuaC).
All the types of memtx tree indexes are supported (multikey, functional).

NO_CHANGELOG=see later commits
NO_DOC=see later commits

Closes #7633
Closes #7636