Hide the last internal utility from the public API

[MateuszKubuszok]

Let's test if private[chimney] fails MiMa check (it should be binary compatible)?

EDIT: it seems it does.

• Result.Errors.__mergeResultNullable is an utility we need for performance (it allow us to avoid allocations, since we can just use var and null when we have no errors to report)
• we cannot remove it for backward compatibility reasons
• but also showing users the method __mergeResultNullable with docs "Please do not use" (which is not defined in internal package), is rather ugly

so we can use some redirection to make it package private - for JVM it is still public but it should no longer be directly usable/no longer shown by IDE.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.67%. Comparing base (0986cda) to head (3415ae6). Report is 122 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #540 +/- ## ========================================== - Coverage 87.72% 87.67% -0.05% ========================================== Files 136 137 +1 Lines 5440 5468 +28 Branches 455 442 -13 ========================================== + Hits 4772 4794 +22 - Misses 668 674 +6 ```

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