A non-blocking database framework for Qt.

FutureSQL was in part inspired by Diesel, and provides a higher level of
abstraction than QtSql. Its features include non-blocking database access by
default, relatively boilderplate-free queries, automatic database migrations
and simple mapping to objects.

In order to make FutureSQL's use of templates less confusing, FutureSQL uses
C++20 concepts, and requires a C++20 compiler.

Warning: The API is not finalized yet.