issue #2852

Why use a strong random number generator in one place, but not another? I know salts have no cryptographic security requirement, but collisions are less likely if you use one.

$cost must be in range 4-31.

prevent call strlen function twice

Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

- Equals sign is now replaced with dot.
- Slash is now replaced with dash.
- Better phpdoc.

`generateSalt` parameter validation fix.

This is a very tiny patch that changes nothing in code logic, but is required to get [AspectMock](https://github.com/Codeception/AspectMock) and Go Aop working with Yii2. 

Go Aop is processing all `include` and `require` directives, replacing them with its filters. Unfortunately it doesn't play well with one-liners. So I had to break the code into few lines to get that working. 

I was trying to fix this issue in [Go Aop](https://github.com/lisachenko/go-aop-php/pull/78) but looks like, the only option is to fix that in Yii2.

AspectMock can dramaticly improve unit testing in Yii2, and I plan to do a blogpost with tutorial about it.