Invariant for default construction

[Martin Nowak via Digitalmars-d]

Walter is about to fix an old bug [1] so that invariants are now called 
before destruction and for non-default construction.

A remaining question is whether invariants should also be called for 
default construction [2].

[1]: https://github.com/D-Programming-Language/dmd/pull/4136
[2]: https://issues.dlang.org/show_bug.cgi?id=519#c11