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 #748.

This does not have the bug #763 since care is taken to avoid allocation unless necessary.

[codecov]

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%)	⬇️
include/boost/json/detail/config.hpp	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce144b5...d854374. Read the comment docs.

[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().