Reimplement iterators using FFI

src/box/bitset_index.cc

src/box/hash_index.cc

src/box/lua/index.cc

src/box/lua/index.h

src/box/lua/schema.lua

src/box/lua/tuple.lua

src/box/tree_index.cc

src/box/tuple.h

src/ffisyms.cc

src/lua/msgpackffi.lua

test/big/hash_multipart.result

test/big/hash_multipart.test.lua

test/big/iterator.result

test/big/lua.result

test/big/sql.result

test/big/sql.test.py

test/big/tree_pk.result

test/big/tree_pk.test.lua

test/box/crossjoin.result

test/box/crossjoin.test.lua