RSRP-469350: Support for C#6.0 `using static` directive

R# at least should:

• Preserve existing using static directive, very important and really annoying. Any cleanup, refactoring and so on, just remove them all
• Has context actions to invoke with using static directive and do not invoke with using static

Context action propose

Lets take a sample code:

using System;

class Program
{
    public static void Main()
    {
         Console.WriteLine("Foo1");
         Console.WriteLine("Foo2");
         Console.ReadKey();
    }
}

Appying the action should produce:

using static System.Console;

class Program
{
    public static void Main()
    {
         WriteLine("Foo1");
         WriteLine("Foo2");
         ReadKey();
    }
}

Plus, the opposed action, Do not invoke as 'using static'.