[IDAG] Pre-Execution Refactoring

[fknorr]

This PR performs some small refactorings necessary for me to introduce the new executor- and backend types in another standalone PR without replacing the entire runtime. It is best reviewed per commit.

1. The additional members attached to device-kernel / host-task instructions for bounds checking now also include the task type, which will improve the quality of diagnostic messages.
2. Instructions and outbound pilots are flushed from the IGGen in batches instead of individually, which reduces the frequency with which an implementor of the delegate (= the executor) needs to take locks to move this info to their thread.
3. accessor and closure_hydrator::accessor_info now refer to "allocations" and "buffers" instead of "virtual" and "backing buffers", and the bounds-checking is improved from using simple arrays to a proper struct.
4. host_task_launcher (currently only used as a placeholder) now is independent of host_queue, which will disappear soon.
5. As the names backend and executor will be re-used, these namespaces / components are renamed to legacy_backend and legacy_executor in the meantime so they can stick around for the next PR.