Add SimpleCell.

[carlosgmartin]

Fixes #3681

[cgarciae]

Thanks @carlosgmartin for doing this! Looks great.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (6181239) 59.02% compared to head (6be118c) 59.10%.
Report is 10 commits behind head on main.

Files	Patch %	Lines
flax/linen/recurrent.py	57.14%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3697      +/-   ##
==========================================
+ Coverage   59.02%   59.10%   +0.07%     
==========================================
  Files         103      103              
  Lines       12391    12454      +63     
==========================================
+ Hits         7314     7361      +47     
- Misses       5077     5093      +16     

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[chiamp]

Thanks!