Use transferrable objects for data transfer

Using `transferList` in `Worker.postMessage()` will reduce memory usage and probably boost performance somewhat by allowing large byte arrays (which we deal with a lot) to be transferred between the worker and the main thread rather than copied.
https://developer.mozilla.org/en-US/docs/Web/API/Worker/postMessage

Start by locating the Worker.postMessage calls that exchange the large byte arrays described in the issue, then read how data moves between the worker and main thread. The work is done when the relevant transfers use transferList without changing existing behavior; check the surrounding tests or runtime checks for memory and performance effects.

Written by the indexing model from the issue text.