Try to extract layer names from Chain, Parallel and SkipConnection

[DrChainsaw]

Fixes #94

This adds functionality to extract layer names from Chain, Parallel and SkipConnection.

This functionality is enabled by default, meaning that if the model to be serialized is a Chain with named layers, ONNXNaiveNASflux will use those names instead of the default ones. The name hunting will recurse to any nested Chains and into Parallels and SkipConnections.

@alicewith: You can see some examples of what this does in the tests, starting here.

Note that the input node is a special node in ONNX which does not exist in a Flux Chain, so just naming the first layer "input" will not give you the result you want; You have to give the name as input to save just as on current master.

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.18%. Comparing base (63d5faf) to head (ad1de43).

Files	Patch %	Lines
src/serialize/namingutil.jl	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   96.12%   96.18%   +0.05%     
==========================================
  Files          15       15              
  Lines        1109     1152      +43     
==========================================
+ Hits         1066     1108      +42     
- Misses         43       44       +1     

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[alicewith]

It looks good! Thank you.

[DrChainsaw]

Alright, just triggered the release process so it should be available soon.