merge Validate clas into ArgumentValidation

[adamsitnik]

use the new Path.Exists method to check for directory and file existance

Initially I wanted to reduce the number of public methods to two:

public static Argument<T> AcceptExistingPathOnly<T>(this Argument<T> argument)
    where T : FileSystemInfo

public static Argument<T> AcceptExistingPathsOnly<T>(this Argument<T> argument)
    where T : IEnumerable<FileSystemInfo>

But since the names are the same, the compiler emits an error.

I've also tried the following:

public static Argument<TC> AcceptExistingPathsOnly<TF, TC>(this Argument<TC> argument)
    where TF : FileSystemInfo
    where TC : IEnumerable<TF>

It works fine, but only when generic type arguments are provided in explicit way ;(