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
You are missing the fact that the code still compiles after the change, albeit with different semantics. If you and @yebblies agree that we should make it an error before allowing it again, then fine with me, but let's not turn this into a giant discussion again.