json_formatter: Make formatter::write work for std::pair

[StephanDollberg]

Previously the un/associative container overloads for formatter::write were broken because it failed to find an overload for write(output_stream, pair).

To fix this we make the write(output_stream<char>&, state, Iter, Iter) overload actually write(output_stream, state, pair) so that the existing overload that handles pair can be found.

Further we fix the fallback write(output_stream, state, T) overload to call formatter::write recursively with the state stripped instead of calling to_json. This keeps the recursive zero-copy nature of formatter::write intact.

All of the above mirrors how the existing overloads for to_json already work.

Adds some tests as well.

[avikivity]

Does the test fail before the patch? If so please reorder them.

[StephanDollberg]

Does the test fail before the patch? If so please reorder them.

Yep, it doesn't compile. Done.