formats_builder::check_references + compose

[tgockel]

Currently, use of formats_builder::check_references is followed by a jsonv::formats::compose with the same other parameter:

jsonv::formats fmts =
    jsonv::formats_builder()
        .type<my_thing>
            .
            .
            .
        .check_references(jsonv::formats::defaults())
    ;
return jsonv::formats::compose({ fmts, jsonv::formats::defaults() });

There should be a new function in the serialization builder DSL called compose_checked which combines these two operations.