Change cells' return to out, state

[CarloLucibello]

This is what proposal 2 in #2548 looks like.
If we do it, we should do it as soon as possible since this requires tagging v0.16.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 32.50%. Comparing base (6041cf5) to head (55fe124).

Files with missing lines	Patch %	Lines
src/layers/recurrent.jl	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2551      +/-   ##
==========================================
- Coverage   32.53%   32.50%   -0.04%     
==========================================
  Files          34       34              
  Lines        2010     2003       -7     
==========================================
- Hits          654      651       -3     
+ Misses       1356     1352       -4     

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