Support "import Foo as Bar" syntax

[stevengj]

Similar to Python, import Foo as Bar would import the Foo module in a variable named Bar, so that you would access the Foo namespace as Bar.foo. This could even be just syntactic sugar for:

import Foo
Bar = Foo

if we don't mind having Foo in the namespace too, but hiding Foo would probably be better.

Useful when you don't want to pollute the global namespace (hence don't want using) but would like a short module name for convenience.

[lindahua]

+1 for this syntax.

[kmsquire]

Dup of #1255.

[pao]

Closing as dup of #1255 per @kmsquire.