[dmd-internals] Pulling D language enhancements

[Walter Bright via dmd-internals]

There's another problem, exhibited by:

https://github.com/D-Programming-Language/dmd/pull/4043
https://github.com/D-Programming-Language/dmd/pull/3998

What both share is that a significant language change is coupled with the code 
to implement it. The trouble is the PR then becomes a mixed-up review of the 
code and the language design change. This makes it hard and confusing to review, and

    -- WORST OF ALL --

if the language design change is rejected, then the PR author's coding effort 
has gone to waste. We don't have the luxury of throwing peoples' work away like 
that. It pains me very much when this happens.

So please, please, before submitting a PR that involves a language change, be 
sure you've got agreement and approval for the change. Otherwise, you risk 
wasting your time in a most frustrating matter.

I'm also OK with others pulling PR implementations for language changes when the 
language changes have already been approved.