basic implementation of context-based path traction

[grisumbras]

Very rough draft of the idea.

For a context type Ctx the user is supposed to implement these functions:

void tag_invoke(context_key_push_tag, string_view,  Ctx const&);
void tag_invoke(context_key_pop_tag, Ctx const&);
void tag_invoke(context_index_push_tag, std::size_t, Ctx const&);
void tag_invoke(context_index_pop_tag, Ctx const&);

The library would invoke them when traversing through the converted/parsed/serialized value.

Things to think about:

• The user may actually want to set error_code in those callbacks.
• The user may want to clear error_code in those functions. But then where would we get the results the calling function was supposed to create?

[cppalliance-bot]

An automated preview of the documentation is available at https://1075.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1075/pullrequest.html

[RoyBellingan]

Is very interesting and clean.
I would suggest in this section

https://github.com/grisumbras/json/blob/e4e192a84f7482aaad923575e34e6f37669252d0/include/boost/json/detail/value_to.hpp#L376

To move the
json::context_key_push(ctx, D::name);

Before the check, so we can capture the "last processed" json pointer