Use UInt64 to track iteration count during warm-up calculation in Benchmark::IPS (#15780)

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: 

```crystal
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.

Co-authored-by: Sijawusz Pur Rahnama <[email protected]>

Author: syeopite

src/benchmark/ips.cr

@@ -65,7 +65,7 @@ module Benchmark
         @items.each do |item|
           GC.collect
 
-          count = 0
+          count = 0_u64
           elapsed = Time.measure do
             target = Time.monotonic + @warmup_time