Request: Unwrap Nullable / Wrap with Nullable

## Unwrap Nullable
I occasionally find code that uses Nullable types when it doesn't need to do so. Unwrapping Nullable by hand can be somewhat tedious.
#### Before
```csharp
private static string CalculateMidpoint(string startTime, string endTime)
{
DateTime? windowStartTime = DateTime.ParseExact(startTime, "HHmm", null);
DateTime? windowEndtime = DateTime.ParseExact(endTime, "HHmm", null);

windowStartTime = windowStartTime.Value.AddMinutes(-windowStartTime.Value.Minute);
windowEndtime = windowEndtime.Value.Minute == 0 ? windowEndtime : windowEndtime.Value.AddMinutes(60 - windowEndtime.Value.Minute);

TimeSpan half = CalculateMidpoint(windowStartTime, windowEndtime);
return (windowStartTime.Value + half).ToString("HHmm");
}
```
#### After
```csharp
private static string CalculateMidpoint(string startTime, string endTime)
{
DateTime windowStartTime = DateTime.ParseExact(startTime, "HHmm", null);
DateTime windowEndtime = DateTime.ParseExact(endTime, "HHmm", null);

windowStartTime = windowStartTime.AddMinutes(-windowStartTime.Minute);
windowEndtime = windowEndtime.Minute == 0 ? windowEndtime : windowEndtime.AddMinutes(60 - windowEndtime.Minute);

TimeSpan half = CalculateMidPoint(windowStartTime, windowEndtime);
return (windowStartTime + half).ToString("HHmm");
}
```
## Reverse: Wrap with Nullable
The ability to promote a local variable from a standard struct/ValueType to a Nullable<> would also be handy. The user should do this with the intention of adding code that would make use of a null. In this case, not having to chase down all of the existing usage to add `.Value` everywhere would be nice. The Before/After examples could be flipped from the above code samples depicting unwrap.

The issue provides C# before/after examples for unwrapping and wrapping Nullable values, but names no files, tests, or entry points. Start by locating the relevant nullable analysis and code-fix areas in Roslynator. Done means both transformations work as described and are covered by appropriate tests.

Written by the indexing model from the issue text.