interfaces and contracts - new pattern

[Adam D. Ruppe via Digitalmars-d-announce]

In short use `in(false)` when you `override` a function to 
inherit the contract, unless you explicitly want to expand the 
input - which you shouldn't do when implementing an interface!

Wrote about it in more details here:
http://dpldocs.info/this-week-in-d/Blog.Posted_2019_12_02.html

i think this is a pretty cool little discovery, thanks too for 
the folks on irc for chatting it through.

destroy if i missed anything lol