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

(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](https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/web/rest.d#L543) (P.R for them to come soon)