Add missing ConstraintExpression.Contain overload

[polsys]

This works:

string str = "a";
Assert.That(str, Does.Contain("a"));
Assert.That(str, Does.Not.Contain("b"));

But this fails to compile (cannot convert from int to string):

int[] ints = { 2 };
Assert.That(ints, Does.Contain(2));
Assert.That(ints, Does.Not.Contain(1));

This PR adds the Contain(object) overload for consistency. The same can be achieved with at least Has.No.Member() and Does.Not.Contains(), so I don't object if this addition is considered unnecessary. This is somewhat related to #1758.

There are no test changes, since the Contain(string) overload is not tested either, as it duplicated work in Contains(), and now just calls it directly.

[CharliePoole]

I'm good with this change. It makes things more consistent. Thanks for doing it.

We need one more committer to approve it before merging.

[ChrisMaddock]

Sounds sensible to me. 👍 Thanks!