Recover ability to initialize `json::value` directly from C++ objects by way of automatically using `value_from`

[pdimov]

As shown in #1125, the natural implementation of value_from for simple structs

void tag_invoke( boost::json::value_from_tag const&, boost::json::value& jv, X const& x )
{
    jv = {
        { "a", boost::json::value_from( x.a ) },
        { "b", boost::json::value_from( x.b ) }
    };
}

is subtly inefficient, even though it compiles and works.

To avoid this trap, we should re-enable the functionality of assigning to json::value directly, as in

void tag_invoke( boost::json::value_from_tag const&, boost::json::value& jv, X const& x )
{
    jv = {
        { "a", x.a },
        { "b", x.b }
    };
}

In addition to being correct, this is also much simpler.

This used to work in 1.80 and was intentionally broken in 1.81 (#823) because "it was never supposed to work." (But it probably was supposed to work, for the reason above.)

Note that the solution proposed in #823 has the same problem as the above example.

[pdimov]

See also #845 and #917.