Support byte[].AsString()

[Tommo-L]

public static string testbytearray2(byte[] a)
{
      Map<string, string> some = new Map<string, string>();
     some[a.AsString()] = "teststring2";
     return Json.Serialize(some);
}

posted at neo-project/neo#1710 (comment)

In upper case, we should convert Buffer to ByteString, as c# byte[] is NeoVM.Buffer, c# string is NeoVM.ByteString. But the compiler just igore AsString method, and return a Buffer.

Three problems:

1. Compiler forget to convert byte[] in argument parameter list to Buffer
2. AsString should convert Buffer to ByteString.
3. Map<Primitive, StackItem>, the key of map can only be Primitive.

[erikzhang]

Map should support string only.

[shargon]

Map should support string only.

Primitive, also should support integer and boolean