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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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