Add == and != operators to ValueTuple.cs

[OzieGamma]

After the following discussion on https://github.com/dotnet/coreclr/issues/6520#issuecomment-236152874 we discovered ValueTuple did not implement the == and != operators.

I think this is an oversight, it looks pretty natural to me to write:

(name, age) = GetPersonTuple();
if ((name, age) == ("Jhon", 13))
{
    // ...
}

OR

tuple = GetPersonTuple();
if (tuple == ("Jhon", 13))
{
    // ...
}

Personally, I think that if the performance cost is not too high I might even compare variables using the tuple notation. It is easier to understand and to read than the && syntax:

var age = AskAge();
var name = AskName();

if((age, name) == (17, "Jhon"))
{
    // ...
}

instead of:

var age = AskAge();
var name = AskName();

if(age == 17 && name == "Jhon")
{
    // ...
}

This might overlap with the Tuple Patterns but I think those were removed from C# 7. Having to call .Equals seems very ugly to me.

tuple = GetPersonTuple();
if (tuple is ("Jhon", 13))
{
    // ...
}

[OzieGamma]

It makes little sense to add these in the CoreFX. (See dotnet/corefx#10417).

I moved the issue to the Roslyn Repo. (dotnet/roslyn#13155)