value_to/from supports std::variant

[grisumbras]

Fix #517

[cppalliance-bot]

An automated preview of the documentation is available at https://771.jsondocs.prtest.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/771/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://771.jsondocs.prtest.cppalliance.org/libs/json/doc/html/index.html

[codecov]

Codecov Report

Merging #771 (6ff132c) into develop (8daa656) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@ Coverage Diff @@ ## develop #771 +/- ## ======================================== Coverage 98.99% 98.99% ======================================== Files 70 70 Lines 6734 6734 ======================================== Hits 6666 6666 Misses 68 68 

Impacted Files	Coverage Δ
include/boost/json/detail/value_from.hpp	100.00% <ø> (ø)
include/boost/json/detail/value_to.hpp	100.00% <ø> (ø)
include/boost/json/value_from.hpp	100.00% <ø> (ø)
include/boost/json/value_to.hpp	100.00% <ø> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/771/pullrequest.html