Issue 3449 - const and invariant struct members do not behave according to spec

https://issues.dlang.org/show_bug.cgi?id=3449

It's time to remove 'implicit static' declaration!

First PR: #93
Warning: #2076
Deprecation: #3038
Ok. I have mixed feelings about this, but if someone has been resolutely compiling with -d long term, I'm not terribly sympathetic.