Optimize MapOf integer hash function

[puzpuzpuz]

Benchmarks for MapOf[int, int] with 64 keys

Before:

BenchmarkIntegerMapOf_WarmUp/reads=100%-8         	170225940	         7.107 ns/op	       0 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=99%-8          	147592124	         8.331 ns/op	       0 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=90%-reads-8    	100000000	        11.09 ns/op	       1 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=75%-reads-8    	78332457	        16.03 ns/op	       4 B/op	       0 allocs/op

After:

BenchmarkIntegerMapOf_WarmUp/reads=100%-8         	208831473	         5.325 ns/op	       0 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=99%-8          	180287820	         6.516 ns/op	       0 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=90%-reads-8    	127232131	         9.812 ns/op	       1 B/op	       0 allocs/op
BenchmarkIntegerMapOf_WarmUp/reads=75%-reads-8    	84348110	        13.95 ns/op	       4 B/op	       0 allocs/op

[psyhatter]

Probably the comment is outdated =)

[puzpuzpuz]

True :)

Thanks for the review! Will fix the comment.