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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (ce144b5) to head (d854374).
⚠️ Report is 314 commits behind head on develop.

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 

Files with missing lines	Coverage Δ
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/object.hpp	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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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().