fixed_point: Mark default constructor as constexpr

Ensures that a fixed-point value is always initialized

This likely also fixes several cases of uninitialized values being
operated on, since we have multiple areas in the codebase where the
default constructor is being used like:

Common::FixedPoint<50, 14> current_sample{};

and is then followed up with an arithmetic operation like += or
something else, which operates directly on FixedPoint's internal data
member, which would previously be uninitialized.