Use UInt64 to track iteration count during warm-up calculation in Benchmark::IPS

[syeopite]

If the warm-up duration is long enough then the iteration count in run_warmup can easily exceed the 32 bit integer limit in a little under a minute resulting in an OverflowError.

This PR just change the tracker variable to an unsigned 64 bit integer so that an overflow is pretty much impossible

You can reproduce the overflow with:

require "benchmark"

# 100 seconds to be safe but I always get an overflow ~ 40 seconds in
Benchmark.ips(warmup: 100.seconds, &.report("") {  }) 

I don't think there's any practical reasons someone would set the warmup duration to be that high but I don't think Benchmark.ips should fail this easily either.