[CUDAX] Add a way to combine thread hierarchies

[pciolkosz]

In order to implement combination of kernel configurations, we first need a way to combine thread hierarchies.

In case of overlap between the hierarchies one of them should have the priority over the other. I decided to implement it as a member function combine on the hierarchy type to make it less symmetrical than a free function. In case of overlap the object on which combine was called has priority over the other hierarchy.

I decided not to support a case where one of the hierarchies is "in the middle" of the other hierarchy. The supported overlap cases are hierarchy ABC with AB, BC or CD, but not just B.

I also decided to do some small cleanups:
can_stack_to_top -> can_rhs_stack_on_lhs because the previous one was confusing about the order
get_first_level_type -> get_first/last_level, because get_first_level_type::level_type seems weird.

[github-actions]

🟩 CI finished in 1h 14m: Pass: 100%/54 | Total: 4h 38m | Avg: 5m 09s | Max: 16m 28s | Hits: 61%/238

• 🟩 cudax: Pass: 100%/54 | Total: 4h 38m | Avg: 5m 09s | Max: 16m 28s | Hits: 61%/238

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    🟩 v100               Pass: 100%/54  | Total:  4h 38m | Avg:  5m 09s | Max: 16m 28s | Hits:  61%/238   
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    🟩 Build              Pass: 100%/49  | Total:  3h 18m | Avg:  4m 03s | Max:  7m 59s | Hits:  61%/238   
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43	linux-amd64-cpu16
5	linux-amd64-gpu-v100-latest-1
4	linux-arm64-cpu16
2	windows-amd64-cpu16

[github-actions]

🟩 CI finished in 1h 27m: Pass: 100%/54 | Total: 4h 44m | Avg: 5m 15s | Max: 20m 26s | Hits: 61%/246

• 🟩 cudax: Pass: 100%/54 | Total: 4h 44m | Avg: 5m 15s | Max: 20m 26s | Hits: 61%/246

  🟩 cpu
    🟩 amd64              Pass: 100%/50  | Total:  4h 30m | Avg:  5m 24s | Max: 20m 26s | Hits:  61%/246   
    🟩 arm64              Pass: 100%/4   | Total: 14m 16s | Avg:  3m 34s | Max:  4m 10s
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    🟩 12.0               Pass: 100%/19  | Total:  1h 42m | Avg:  5m 22s | Max: 20m 26s | Hits:  61%/123   
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    🟩 GCC                Pass: 100%/20  | Total:  1h 54m | Avg:  5m 44s | Max: 20m 26s
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    🟩 Build              Pass: 100%/49  | Total:  3h 22m | Avg:  4m 07s | Max:  9m 30s | Hits:  61%/246   
    🟩 Test               Pass: 100%/5   | Total:  1h 22m | Avg: 16m 26s | Max: 20m 26s
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    🟩 20                 Pass: 100%/25  | Total:  2h 24m | Avg:  5m 46s | Max: 17m 04s | Hits:  61%/246   
  

👃 Inspect Changes

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+/-	CUDA Experimental
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	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
+/-	CUDA Experimental
	python
	CCCL C Parallel Library
	Catch2Helper

🏃‍ Runner counts (total jobs: 54)

#	Runner
43	linux-amd64-cpu16
5	linux-amd64-gpu-v100-latest-1
4	linux-arm64-cpu16
2	windows-amd64-cpu16