Provide a way to alias namespaces

TypeScript should have a way to embed (type) namespaces. In the following case, assigned (embeded) namespace NS.A should have a C type.

TypeScript Version: master

Code

namespace NS_A {
  export class C {
  }
}
namespace NS {
  export var A = NS_A;
  export type A = NS_A;
}
var C = NS.A.C;
type C = NS.A.C;

Expected behavior:

$ node built/local/tsc.js --lib es6 -m commonjs --noEmit index.ts

Actual behavior:

$ node built/local/tsc.js --lib es6 -m commonjs --noEmit index.ts
index.ts(7,19): error TS2304: Cannot find name 'NS_A'.
index.ts(10,13): error TS2305: Module 'NS' has no exported member 'A'.

Start with the reproducer in index.ts and run the issue's built/local/tsc.js command to confirm the namespace and type errors. Trace the compiler's handling of namespace aliases and exported types, then define the expected behavior for both the value expression and type reference. Done means the sample compiles without the reported errors.