rtree: reuse Rect instances to reduce allocations

In my 3M 2D-segments insertion benchmark, the allocations and related GC
collections were on top. Creating single rectangle instances and reusing
them in loops brings the test from 3mn12s to 2mn6s.
geom: inline Min/Max in Rect.enlarge

In my 3M segments insertion benchmark, it reduces total time from 2mn6s
to 1mn40s.
rtree: inline boundingBoxN into computeBoundingBox

This brings my 3M segments insertion benchmark from 1mn40s to 1mn31s.
Merge pull request #1 from pmezard/performance-improvements

Performance improvements