Add experimental task hints API, support for 2D splits

[psalz]

This adds a new experimental::hint API that can be used to provide the runtime with additional information on how to process a task. The first two hints we have are experimental::hints::split_1d and experimental::hints::split_2d. As a 1D split is currently the default, the former is a no-op. For the latter this introduces a new split_2d function that attempts to produce a number of evenly sized chunks while satisfying the provided granularity constraints.

The algorithm works by trying to find two factors for num_chunks, the number of requested chunks, that are as close to sqrt(num_chunks) as possible while also producing the requested number of chunks, degrading to a 1D split in the worst case. Due to granularity constraints it is not always possible to produce the requested number of chunks however, in that case the algorithm will attempt to get as close as possible. Finding the factors requires at most O(sqrt(num_chunks)) time.

Given the two factors, the algorithm uses two heuristics to decide to which side of the domain they should be assigned:

• If the domain is square-ish, the algorithm prefers to have more splits along the slower dimension.
• If the domain is non-square, the algorithm prefers the split that produces overall shorder edges (smaller sum of circumferences).

Here's a visual representation of some of the 2D splits from the unit tests. Each split is labelled in the form <size0>x<size1> ~ <granularity0>x<granularity1> | <num chunks>. Red lines represent split constraints (i.e., individual red boxes can not be split further):

[github-actions[bot]]

Check-perf-impact results: (3b34e58e3c100f4c3541a1ed59580f72)

:question: No new benchmark data submitted. :question:
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[fknorr]

I noticed that task_manager::submit_command_group has a hints parameter, I guess that's obsolete now.

[github-actions[bot]]

Check-perf-impact results: (4c65f1399a47e0eb1340f63004745b17)

:question: No new benchmark data submitted. :question:
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.