Some String support

A simple program as this one:

 static void Main()
 {
        var s1 = "Hello";
        var s2 = " world";
        var s3 = s1 + s2;
        Console.Write(s3);
 }

 would not work and even seems to be a simple program, in fact is a bit harder because CR didn't have a nice way to map types and to implement a mix of the CR's internal string and the real string, so there was the need to duplicate the entire code of string and make it work.

Right now it works as the following:

[ExtensionsImplementation(typeof (string))]
    public static class StringImpl
    {
        [MapMethod]
        public static string Substring(string _this, int startIndex)
        {(...)}
        [MapMethod(IsStatic = true)]
        public static string Concat(string s1, string s2)
        {(...)}
        [MapMethod]
        public static char[] ToCharArray(CrString _this)
        {(...)}
 }
By adding implementation of mapped CrString for the System.String type makes possible to make implementations natural and as the library will grow (please spend very little time to support a simplified version of any class you think you can implement in .Net) many programs which were missing just the part of displaying some combined strings, right now is all possible.