caller provided serializer storage

[grisumbras]

Fix #844

[codecov]

Codecov Report

Merging #848 (c06b17a) into develop (d7b7bef) will increase coverage by 0.19%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #848      +/-   ##
===========================================
+ Coverage    99.00%   99.19%   +0.19%     
===========================================
  Files           70       70              
  Lines         6837     6849      +12     
===========================================
+ Hits          6769     6794      +25     
+ Misses          68       55      -13     

Impacted Files	Coverage Δ
include/boost/json/serializer.hpp	100.00% <ø> (ø)
include/boost/json/detail/impl/stack.ipp	100.00% <100.00%> (ø)
include/boost/json/detail/stack.hpp	100.00% <100.00%> (ø)
include/boost/json/impl/serialize.ipp	94.93% <100.00%> (+7.59%)	⬆️
include/boost/json/impl/serializer.ipp	98.62% <100.00%> (+1.64%)	⬆️

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/848/pullrequest.html