Allow forcing loop unrolling on a per-loop basis

[Speedphoenix]

There are cases where you want a loop to be unrolled regardless of cost (eg. to avoid allocation on an array in performance-sensitive functions.) Such cases may not warrant increasing -D loop_unroll_max_cost for the whole project.

Something like @:unroll or @:forceUnroll before a loop that guarantees unrolling or an error otherwise would help in these cases.

[kLabz]

Maybe both @:unrollCost(X) for local override of -D loop_unroll_max_cost, and @:forceUnroll?

[Simn]

I was thinking inline for(...) could be a thing, but I haven't thought this through.

I'm fine with a metadata to override this though.