sqlpp::optional is an alias for std::optional for C++17 and beyond.
Otherwise, it is a simple and incomplete backport.
For older versions of C++, the library offers simple back
Similar for string_view and span.
Removed timezone handling in the process (needs to be documented).
Note: on_conflict does not check for constraints (needs to be documented).
Note: some of the constraints tests seem to be aiming for a different static_assert.
