updates for Functors v0.5

[CarloLucibello]

Adapting docs and the layer macro to Functors v0.5

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 33.54%. Comparing base (e2f74bf) to head (fe46c6f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layers/show.jl	0.00%	3 Missing ⚠️
src/outputsize.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2528      +/-   ##
==========================================
+ Coverage   33.49%   33.54%   +0.05%     
==========================================
  Files          31       31              
  Lines        1908     1881      -27     
==========================================
- Hits          639      631       -8     
+ Misses       1269     1250      -19     

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• Flaky Tests Detection - Detect and resolve failed and flaky tests

[mcabbott]

To be rebased on top of #2509?

[CarloLucibello]

To be rebased on top of #2509?

yes I would have done that from the beginning if I wasn't too lazy to rebase on a PR from a fork. I will cherry-pick from #2509 and add the co-author message when squashing and merging.