Make source a nested type of serializer

[ashtum]

No description provided.

[vinniefalco]

type traits twice?

[cppalliance-bot]

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Codecov Report

Attention: Patch coverage is 97.77778% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.09%. Comparing base (39a71bf) to head (ef7098e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #74   +/-   ##
========================================
  Coverage    88.09%   88.09%           
========================================
  Files           78       75    -3     
  Lines         4260     4260           
========================================
  Hits          3753     3753           
  Misses         507      507           

Files	Coverage Δ
include/boost/http_proto/serializer.hpp	78.57% <100.00%> (+16.07%)	⬆️
src/serializer.cpp	74.89% <100.00%> (+2.37%)	⬆️
include/boost/http_proto/impl/serializer.hpp	95.74% <94.44%> (-0.81%)	⬇️

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[ashtum]

Decided to keep the source in the namespace, as moving it to a nested type of serializer would cause every source type to include serializer.hpp.
Closes #70