Simplify Avro marshaller

[cb372]

What this does?

Simplify the code for parsing an incoming request as Avro.

Checklist

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

[codecov]

Codecov Report

Merging #740 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
+ Coverage   81.34%   81.39%   +0.05%     
==========================================
  Files          73       73              
  Lines        1163     1161       -2     
  Branches       15       14       -1     
==========================================
- Hits          946      945       -1     
+ Misses        217      216       -1

Impacted Files	Coverage Δ
...higherkindness/mu/rpc/internal/encoders/avro.scala	82.02% <100%> (-0.4%)	⬇️
...kindness/mu/rpc/idlgen/avro/AvroSrcGenerator.scala	76.66% <0%> (+1.66%)	⬆️

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