[Grace-core] Module exports

[Andrew P. Black]

Interesting discussion of modules in the context of Klau's talk here at WG 2.16.  It's important for a module system to be able to export as well as import. Java lacks this.  An export facility lets you write a module whose role is to assemble bits from other modules, repackage them, add to them, and then export the parts of interest as a new module.

We should consider adding exports to Grace modules.  In fact, it may be that the difference between a Grace object and a Grace module is exactly that the module exports stuff (still as an object, perhaps.)

	Andrew