index: rearrange require calls
use "bounding-client-rect" module

This gives a slight performance boost, since before we would
potentially be calling `getBoundingClientRect()` twice.

Now the TextRectangle is being re-used, and works with more
types of Nodes. So now `component/tip` should be compatible
with regular DOM Elements, TextNode instances and Ranges.
Merge pull request #49 from component/update/use-bounding-client-rect-module

Use "bounding-client-rect" component