champ for_each_chunk_p

[fabianbs96]

Currently, the immer set, map, and table containers only support a subset of available algorithms; especially immer::all_of is not supported. That is, because the underlying champ does not implement for_each_chunk_p.

This PR adds an implementation of for_each_chunk_p to the above mentioned champ. Should be related to #171.

Design considerations:

• iterative- vs recursive implementation: As `for_each_chunk_p may be part of extremely hot parts of user-code, I prefer to avoid non-tail recursion
• Worklist vs explicit stack: The easiest non-recursive implementation would be based on a worklist of const node_t *. However, this has the drawback of potential memory allocation, so the implementation uses an explicit stack (std::array) that models the required parts of the call-stack from for_each_chunk_traversal.
• Question: Should we std::invoke the callback if compiled with C++17 or higher?

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5875f77) 90.53% compared to head (94dc1fc) 90.54%.

:exclamation: Current head 94dc1fc differs from pull request most recent head 6cd57a2. Consider uploading reports for the commit 6cd57a2 to get more accurate results

Files	Patch %	Lines
immer/detail/hamts/champ.hpp	86.66%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #270 +/- ## ========================================== + Coverage 90.53% 90.54% +0.01% ========================================== Files 119 119 Lines 12144 12203 +59 ========================================== + Hits 10994 11049 +55 - Misses 1150 1154 +4 ```

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