Merge with master
Introduce BOX key type for R-Tree
Add unit tests for R-Tree
Add unit tests for R-Tree
converted rtree to plain c, moved it to salad
Merge remote-tracking branch 'origin/master' into rtree-v4
brought back area_t definition, added sq_coord_t type, avoided DBL_MAX dependance
added several comments to rtree.h
Merge branch 'rtree-v4' of github.com:tarantool/tarantool into rtree-v4
Review fixes (rtree-v4).
rtree-v4, review fixes

Weed out 'box' data type for now. A special name can backfire
in the future, when we exteend HASH/TREE/other types.

Add checks for uniqueness.

Disable check of key parts, it was not used any way,
since an explicit check is made in initIterator() and findByKey().

Mark sophia engine as  not supporting RTREE.