relax iterator requirements

[grisumbras]

Fix #840

[codecov]

Codecov Report

Merging #919 (8e5eed3) into develop (0a7860f) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #919   +/-   ##
========================================
  Coverage    92.95%   92.95%           
========================================
  Files           85       85           
  Lines         8047     8047           
========================================
  Hits          7480     7480           
  Misses         567      567           

Impacted Files	Coverage Δ
include/boost/json/array.hpp	100.00% <ø> (ø)
include/boost/json/object.hpp	100.00% <ø> (ø)
include/boost/json/string.hpp	100.00% <ø> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/919/pullrequest.html