QDate construction from std chrono types is inefficient

The conversion constructors are are all essentially implemented by constructing a QDate that represents the Unix Epoch, and then calling addDays() on it.

The problem is that addDays() is an out-of-line function and therefore the whole conversion isn't constexpr.

Both types involved are Literal Types. It's not acceptable that the conversion between them is not constexpr.

Acceptance criteria:

• all conversions from QDate to std::chrono types, and vice versa, are constexpr
• proven by tests