Bring vibe.d's enforce closer to std.exception.enforce

[Geod24]

(1) Use of lazy const(char)[] instead of string -> No more eager format() call;
(2) Return the value computed;

There are some low-hanging fruit for (2), like here (P.R for them to come soon)

[s-ludwig]

Thanks, makes sense. The only thing that I'm not sure about is using const(char)[]. Does the original enforce have to .idup the message, too?

[Geod24]

Yes it does:
https://github.com/D-Programming-Language/phobos/blob/master/std/exception.d#L370

[s-ludwig]

That's ugly :(

In that case I'd propose to make a second overload that takes a lazy string, because in all cases that I know of the input string is already immutable.

[Geod24]

Well, const(char)[] isn't really needed. And I don't want to multiply pointless overloads, so lazy string it is.

[s-ludwig]

Alright, that's fine by me, too. Merging.