Tracking Issue for slice::retain

[MarinPostma]

This proposes to add a retain method to slice, that does the same thing as Vec::retain.

This tracking issue, and it's ascociated PR(#71832) are heavily influenced by the discussion in #54279

[KodrAus]

Since the implementation PR was closed (just due to inactivity) we don't really have a feature to track here. I'll go ahead and close this one now, but if #71832 returns in some form then feel free to re-open!