index: convert to plain struct

src/box/index.cc

src/box/index.h

src/box/memtx_bitset.cc

src/box/memtx_bitset.h

src/box/memtx_hash.cc

src/box/memtx_hash.h

src/box/memtx_rtree.cc

src/box/memtx_rtree.h

src/box/memtx_space.cc

src/box/memtx_tree.cc

src/box/memtx_tree.h

src/box/space.cc

src/box/sysview_engine.cc

src/box/sysview_index.cc

src/box/sysview_index.h

src/box/vinyl_index.cc

src/box/vinyl_index.h

src/box/vinyl_space.cc