Make Str.chomp a bit smarter

1. don't change anything if there's nothing to chomp
   doing 1M chomps on a short string: 1.534 -> 1.138
2. reduce number of checks and characters checked
   1M x Foo\n:   1.749 -> 1.717
   1M x Foo\r\n: 1.842 -> 1.721