vibe.d & exception performance

[Dicebot]

You seem to forget that actually writing data to socket takes 
time and this time is longer than one needed for most simple 
stack unwinding. Fun fact - even if you copy rendering code to 
static exception example it will still be ~5-10% faster because 
it will cancel pending async socket output and terminate fiber 
early.

But yes, those results match my earlier observations - 
pre-allocated exceptions with no stack trace generation are of 
fast enough to not be a concern. Impact of both of those factors 
is huge enough to provide DoS vector.