[BUGFIX beta] Clean up TrackedArray.

- Remove incorrect use of "transformation"
- s/_content/_operations for clarity
- s/operation.operation/operation.type for clarity
- some crazy comment formatting
- added TrackedArray#toString.  Mostly helpful in debugging.
[BUGFIX beta] Fix edge case in `TrackedArray`.

Fixes http://jsbin.com/EFezAP/2

Fix an edge case in delete composition for a delete that right-trims an insert.
This results in a delete followed by an insert that should both compose to a
no-op, but the insert was being erroneously left behind.

Also ignore no-op adds and deletes (ie add 0 items, delete 0 items).
Merge pull request #3533 from hjdivad/fix-trackedarray-bug

[BUGFIX beta] Fix edge case in `TrackedArray`.