* Move UDLs into nlohmann::literals::json_literals namespace
* Add 'using namespace' to unit tests
* Add 'using namespace' to examples
* Add 'using namespace' to README
* Move UDL mkdocs pages out of basic_json/
* Update documentation
* Update docset index
* Add JSON_GlobalUDLs CMake option
* Add unit test
* Build examples without global UDLs
* Add CI target
When parsing a string containing two JSON values using iterators, after
parsing, iterator_input_adapter should have advanced to the first
character of the second value.
Add a unit test to check that this is true.
