Proposal: Improved type aliases

[alrz]

Currently we can define type aliases as follow
using Cache = System.Collections.Generic.Dictionary<string, string>;
but it does not support generic types and needs to be written with full namespaces
using Cache<T> = Dictionary<string, T>;
and also it is only in the file scope.

In the context of DDD, it would be a useful feature to have generic and universal type aliases so we can write methods and extension methods for specific yet simple types like
using EmailAddress = string;

[HaloFour]

Duplicate combining #58, #104, #116, #2044 and #3993.

[gafter]

Looks a great deal like #3993.