converted rtree to plain c, moved it to salad

cmake/BuildMisc.cmake

src/box/rtree_index.cc

src/box/rtree_index.h

src/lib/salad/CMakeLists.txt

src/lib/salad/bps_tree.h

src/lib/salad/rtree.c

src/lib/salad/rtree.h

test/box/rtree_benchmark.result

test/box/rtree_benchmark.test.lua

test/box/rtree_rect.result

test/box/rtree_rect.test.lua

test/unit/CMakeLists.txt

test/unit/rtree.cc

test/unit/rtree_itr.cc

third_party/rtree.cc

third_party/rtree.h