feat: Add PriorityQueue

[Andrei1998]

This PR adds PriorityQueue support to the core library, implementing two variants:

1. Binary heap-based implementation using List (under src/PriorityQueue.mo).
2. Wrapper over Set (under bench/utils/PriorityQueueSet.mo and intended for benchmarking/testing purposes only).

Resolves #391.

Tests

The PR contains comprehensive unit tests, including cross-testing the two implementations against each other to ensure correctness, as well as more complex test scenarios such as long randomized sequences of operations and exhaustive testing of all possible small sequences to catch edge cases.

Benchmarks

The PR contains extensive benchmarks comparing the performance of both implementations (on Nat values). See table below.

Note: an earlier commit in this PR considered simpler but less efficient ways of implementing push and pop for the heap-based implementation. These were removed in favor of more efficient implementations based on the gap technique to avoid needless copies. Concretely, commit 664d092 distinguished between:

• push (removed) and pushBetter (now renamed push).
• pop (removed) and popBetter (now renamed pop).

The table below is for the state of the code at this commit.

---

Instructions

	A) PriorityQueue	A2) PriorityQueue better push	A3) PriorityQueue better push, better pop	B) PriorityQueueSet
1.) 100000 operations (push:pop = 1:1)	1_078_836_395	871_391_356	598_903_656	522_729_799
2.) 100000 operations (push:pop = 2:1)	1_365_419_609	1_081_007_251	744_336_786	809_693_353
3.) 100000 operations (push:pop = 10:1)	598_673_615	461_615_949	359_307_559	873_181_417
4.) 100000 operations (only push)	271_131_449	193_823_567	193_823_107	886_825_181
5.) 50000 pushes, then 50000 pops	1_347_356_950	1_308_569_002	778_032_800	961_777_046
6.) 50000 pushes, then 25000 "pop;push"es	902_261_756	813_912_597	530_870_481	922_137_746

Heap (@alexandru-uta noted that these seem broken at the moment, likely due to independent reasons):

	A) PriorityQueue	A2) PriorityQueue better push	A3) PriorityQueue better push, better pop	B) PriorityQueueSet
1.) 100000 operations (push:pop = 1:1)	308 B	272 B	272 B	272 B
2.) 100000 operations (push:pop = 2:1)	308 B	308 B	272 B	272 B
3.) 100000 operations (push:pop = 10:1)	272 B	272 B	272 B	272 B
4.) 100000 operations (only push)	272 B	272 B	272 B	272 B
5.) 50000 pushes, then 50000 pops	308 B	308 B	272 B	272 B
6.) 50000 pushes, then 25000 "pop;push"es	272 B	272 B	272 B	272 B

Garbage Collection

	A) PriorityQueue	A2) PriorityQueue better push	A3) PriorityQueue better push, better pop	B) PriorityQueueSet
1.) 100000 operations (push:pop = 1:1)	10.23 MiB	15.03 MiB	15.03 MiB	17.43 MiB
2.) 100000 operations (push:pop = 2:1)	12.91 MiB	19.73 MiB	19.73 MiB	19.32 MiB
3.) 100000 operations (push:pop = 10:1)	4.22 MiB	8.67 MiB	8.67 MiB	12.64 MiB
4.) 100000 operations (only push)	406.75 KiB	3.87 MiB	3.87 MiB	9.96 MiB
5.) 50000 pushes, then 50000 pops	20.29 MiB	22.03 MiB	22.03 MiB	26.2 MiB
6.) 50000 pushes, then 25000 "pop;push"es	11.05 MiB	14.22 MiB	14.22 MiB	18.44 MiB

[rvanasa]

Thank you for the PR! Added a few comments.

[rvanasa]

Should be ready to merge after updating the *Better functions. Thanks again!