Introduce BOX key type for R-Tree

src/box/engine_memtx.cc

src/box/index.cc

src/box/key_def.cc

src/box/key_def.h

src/box/rtree_index.cc

test/box/rtree_array.result

test/box/rtree_array.test.lua

test/box/rtree_benchmark.result

test/box/rtree_benchmark.test.lua

test/box/rtree_point.result

test/box/rtree_point.test.lua

test/box/rtree_point_r2.result

test/box/rtree_point_r2.test.lua

test/box/rtree_rect.result

test/box/rtree_rect.test.lua

test/box/suite.ini

third_party/rtree.cc

third_party/rtree.h