Update all examples and results for fast comes first

Update template:

```
require 'benchmark/ips'

def fast
end

def slow
end

Benchmark.ips do |x|
x.report('fast') { fast }
x.report('slow') { slow }
x.compare!
end
```

and reorganize all stuff according to :point_up_2: :point_up_2: :point_up_2:

Since `x.compare!` will show fast then slow, this may have a better reading experience when reading README IMO. Also document in CONTRIBUTING.md to give people notice when submits a new entry.