feat: Allow repeated scopes

[dsarlis]

This change allows for repeated scopes to be used in canbench benchmarks. Previously the code would straight up refuse to have a similarly named scope -- I think this is a mistake, there are very valid scenarios (example case from stable structures) where someone might have a loop and they want to declare a scope inside it to measure some operation.

People should already be careful not declaring the same names for unrelated scopes, so I don't think a protection against it is worth it (that's I guess the main reason for this restriction currently) and the extra benefit you get to measure things inside loops (which can usually be where heavier parts of processing happens) outweighs some risk that someone re-uses the same name of scope.

[github-actions]

canbench 🏋 (dir: examples/btreemap_vs_hashmap)

No significant performance changes detected ✅

examples/btreemap_vs_hashmap/canbench_results.yml is up to date ✅

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Benchmark: insert_users
  total:
    instructions: 2.56 B (no change)
    heap_increase: 872 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: remove_users
  total:
    instructions: 2.11 B (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: pre_upgrade_bench
  total:
    instructions: 438.88 M (0.00%) (change within noise threshold)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 184 pages (no change)

  serialize_state (scope):
    instructions: 423.95 M (no change)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 0 pages (no change)

  writing_to_stable_memory (scope):
    instructions: 14.93 M (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 184 pages (no change)

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[github-actions]

canbench 🏋 (dir: examples/fibonacci)

No significant performance changes detected ✅

examples/fibonacci/canbench_results.yml is up to date ✅

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Benchmark: fibonacci_20
  total:
    instructions: 726 (-0.55%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: fibonacci_8a
  total:
    instructions: 450 (-0.88%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: fibonacci_8b
  total:
    instructions: 207 (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: fibonacci_8c
  total:
    instructions: 391 (-1.01%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: fibonacci_45
  total:
    instructions: 1301 (-0.31%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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