Improve bitmap slice unchecked

[Dousir9]

if a Bitmap is all true or all false, we don't do a bitcount in slice_unchecked.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (6271f48) 83.06% compared to head (7af8160) 83.05%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1574 +/- ## ========================================== - Coverage 83.06% 83.05% -0.02% ========================================== Files 391 391 Lines 42898 42901 +3 ========================================== - Hits 35635 35631 -4 - Misses 7263 7270 +7 ``` | [Files](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) | Coverage Δ | | |---|---|---| | [src/bitmap/immutable.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2JpdG1hcC9pbW11dGFibGUucnM=) | `85.58% <100.00%> (-0.26%)` | :arrow_down: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1574/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao)

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