[Proposal] Local and generic aliases for types

I came across this vb code:
```VB.NET
Public ReadOnly Property Includes As List(Of Expression(Of Func(Of T, Object))) =
New List(Of Expression(Of Func(Of T, Object)))()
```

and I said: this complexity can't be a VB code!
And I wondered: Can't I simplify this to:
```VB.NET
Imports Expr = Expression(Of Func(Of T, Object))

Public ReadOnly Property Includes As List(Of Expr) =
New List(Of Expr)()
```

This seems more like VB code.
But unfortunately it is not possible, since T exists only inside the class scope, and Imports exists only in file/namespace scope!

So, I wish if VB allows importing type aliases inside classes (the first lines after inherits and implements). This is a must in today's projects that contain very complex generics and lambdas that make our heads spin.

Another good idea, is to allow declaring generic aliases at the file level:
```VB.NET
Imports Expr(Of T) = Expression(Of Func(Of T, Object))
```

So, we can use it in the class like this
```VB.NET
Class Foo(of T)
Public ReadOnly Property Includes As List(Of Expr(Of T)) =
New List(Of Expr(Of T))( )
End Class
```

This is a bit longer, but we can use Expr(Of T) in all classes in the same file.