compatible copy and move conversions for `basic_result<>`. This lets you construct
a `basic_result<T, E>` into a `basic_result<T, error_code>`, where `E` is a
custom type which has implemented the ADL discovered free function
`error_code make_error_code(E)`, but is otherwise unrelated to `error_code`.
The same availability applies for `exception_ptr` with `make_exception_ptr()` being
the ADL discovered free function. `basic_outcome<>` has less support for this than
`basic_result<>` in order to keep constructor count down, but it will accept via
this mechanism conversions from `basic_result<>` and `failure_type<>`.
