Add WORKER_LOADER.load() for one-off workers.
`LOADER.load(code)` is equivalent to `LOADER.get(null, () => code)`.
This change got annoying to the pesky detail that the system might call the callback multiple times, forcing me to create clone() methods for a bunch of structures so that I could build the structure ones and return a clone on every call. The source code itself is refcounted between these clones, at least.