speeding up + ctfe question

[maarten van damme]

well, I was thinking about using a sieve but when you were to request
prime 400_000 you're sieve would blow up in size. That's why I opted
for something else (and I don't know if it was the right thing to do
though). (Ab)using opIndex like that is indeed a bit wrong but what
would be the alternative? remove slicing and introduce getPrime?
Wouldn't that be the same but without the great syntactic sugar?