Make Assert.IsNotNull generic and have it return the value that was checked

## Summary

I'm 120% sure that the boat has long sailed for this proposal (with probably very good reasons, such as code side explosion), but I'll try anyway.

It would've been nice if the `Assert.IsNotNull()` overloads where generic and returned the value that was passed to it.
This would've allowed for nice code like this:
```c#
var fi = new System.IO.FileInfo(@"c:\temp\x.log");
var dir = Assert.IsNotNull(fi.Directory);
Assert.AreEqual("temp", dir.Name);
```

Start by locating the existing Assert.IsNotNull overloads in the MSTest framework and review how assertion APIs are organized. Assess the proposed generic return behavior and overload implications; done means the supported API behavior is defined and validated without an unmanageable overload expansion.

Written by the indexing model from the issue text.