Scalameta codegen

[cb372]

What this does?

Bump the skeuomorph dependency and use its new scalameta-based printer for source code generation.

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #746 into master will decrease coverage by 0.22%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
- Coverage   81.39%   81.17%   -0.23%     
==========================================
  Files          73       73              
  Lines        1161     1158       -3     
  Branches       14       14              
==========================================
- Hits          945      940       -5     
- Misses        216      218       +2

Impacted Files	Coverage Δ
...ndness/mu/rpc/idlgen/proto/ProtoSrcGenerator.scala	82.35% <71.42%> (-7.65%)	⬇️
...kindness/mu/rpc/idlgen/avro/AvroSrcGenerator.scala	75% <0%> (-1.67%)	⬇️

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[cb372]

The coverage went down but it's pretty much impossible to test the error-handling branch. An error in this case basically means a bug in skeuomorph.