vibe.d & exception performance

[Marc Schütz" <schuetzm at gmx.net>]

> The results:
>
>                          A) vibe.d 0.7.17      B) vibe.d patched
> 1) no exceptions              13000 req/s          13000 req/s
> 2) dynamic exceptions           200 req/s            800 req/s
> 3) static exceptions            320 req/s          19000 req/s

New numbers with DMD/Druntime/Phobos git (includes the patch by 
Adam Ruppe):

                          A) vibe.d 0.7.17      B) vibe.d patched
1) no exceptions              15000 req/s          16000 req/s
2) dynamic exceptions           120 req/s          19000 req/s
3) static exceptions            120 req/s          19000 req/s

Dynamic and static exceptions are now equally fast, provided 
`toString()` is not called on them.