Switched to BTreeMap and BTreeSet

[bushrat011899]

Objective

For no_std compatibility it would be useful to move away from HashMap and HashSet, and instead use either hashbrown or alloc::collections::BTreeMap/Set. Since the cases where HashMap and HashSet are used are compatible with BTreeMap and BTreeSet, we should use those instead of introducing a new dependency.

Solution

• Replaced std::collections::HashMap with alloc::collections::BTreeMap
• Replaced std::collections::HashSet with alloc::collections::BTreeSet
• Confirmed no performance change using cargo bench rgb_frame (the only benchmark that hits this code-path).

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Notes

On my Intel i5-1240p laptop I observed no difference in performance due to this PR:

rgb_frame/test.png      time:   [43.728 ms 44.277 ms 44.856 ms]
                        thrpt:  [116.15 MiB/s 117.67 MiB/s 119.15 MiB/s]
                 change:
                        time:   [-1.9516% -0.2032% +1.6099%] (p = 0.82 > 0.05)
                        thrpt:  [-1.5844% +0.2036% +1.9904%]
                        No change in performance detected.

It's possible other platforms may show a more pronounced performance delta, but without evidence I think the tradeoff between possible performance loss vs increased platform compatibility is worth it here.

[bushrat011899]

See #206 for CI failure details.