object::stable_insert

[gummif]

This is for inserting values at any position in the object preserving order of all elements (at the cost of linear complexity). Part 2/2 of https://github.com/boostorg/json/issues/748.

This does not have the bug https://github.com/boostorg/json/issues/763 since care is taken to avoid allocation unless necessary.

[codecov[bot]]

Codecov Report

Merging #765 (d854374) into develop (ce144b5) will decrease coverage by 0.16%. The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #765      +/-   ##
===========================================
- Coverage    99.13%   98.97%   -0.17%     
===========================================
  Files           69       69              
  Lines         6587     6623      +36     
===========================================
+ Hits          6530     6555      +25     
- Misses          57       68      +11     

Impacted Files	Coverage Δ
include/boost/json/object.hpp	100.00% <ø> (ø)
include/boost/json/impl/object.hpp	100.00% <100.00%> (ø)
include/boost/json/impl/object.ipp	100.00% <100.00%> (ø)
include/boost/json/basic_parser_impl.hpp	98.33% <0.00%> (-0.88%)	:arrow_down:
include/boost/json/detail/config.hpp	100.00% <0.00%> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/765/pullrequest.html

[gummif]

One thing that could reduce duplication is to implement insert in terms of stable_insert_impl using pos as end().