Performance of loops

[via Digitalmars-d]

Most of the time is taken up by the array's allocation. The 
optimizers of both DMD and LDC evidently doesn't optimize it away 
when you use `ref`, even though it could in theory.

Remove the `enum` and just use a normal global variable, and you 
will get more or less identical times for all three loops. LDC 
even turns then into empty functions (didn't check for DMD).