The normalizeOS function is quite used, so it's time to optimize it.

By doing it with a bare switch instead of a fancy map, we have an average
speedup of 3x. I've made a benchmark for this specific function. The benchmark
consisted of calling this function 10000 times. I've got the following results:

  mssola:~ $ go run main.go
  Average Time 1 (old): 7.561771ms
  Average Time 2 (new): 2.512584ms
  It's 3.009559481394453 times faster.