Generic using alias directives

[naine]

Currently it is possible to create file-scoped type aliases in C#.

using Foo = System.Collections.ArrayList;

For generic types, it is currently only possible to create aliases to constructed types.

using Foo = System.Collections.Generic.List<int>;

I propose to allow the alias itself to be generic, using syntax like the following:

using Foo<T> = System.Collections.Generic.List<T>;
using Bar<T> = System.Collections.Generic.Dictionary<string, T>;
using Bar<T1, T2> = System.Collections.Generic.Dictionary<T1, T2>;

[HaloFour]

See: #1239