Introduce glob imports in import statements

[sezna]

branched off of #1469

This PR mainly introduces glob imports in import statements, e.g.:

import Foo.*;
import Foo.Bar.* as Baz;

et cetera.

Glob imports work similarly to other languages: Foo.* brings in everything from Foo, while Foo.* as Bar brings in all of Foo, but aliased under a namespace Bar. For example, if function F() : Unit {} exists in Foo, then after importing via Foo.* as Bar, it would be available as Bar.F().

Other incidental changes include:

• Improving the Display impl for ImportOrExportItem, for easier-to-read tests