Kill getOffsetFromNode() with fire

The way it operated was evil. It did a depth-first search from the root,
finding the node using reference equality. For documents with deep
structures, this could take a long time. Inez did some profiling and
found it was called tens of millions of times on a complex document.

Kill getOffsetFromNode() and move its functionality to getOffset().
The logic has been completely rewritten: getOffset() now traverses
up from the node rather than down from the root, and pretty much does
the reverse of what getNodeFromOffset() does. This should be much more
efficient even without offset caching in the node objects (which we may
still implement later).

Change-Id: I125f9fa423c40db6472e2c4a7c94214218ba3bc7