Instance: add exports() helpers for all exportable members

[evacchi]

Currently Instance only exposes export("name") for function. We should also export tables, globals, memory

Instead of polluting the class namespace with multiple exportGlobal, exportTable, etc. we propose to instead have an .exports() with multiple members, e.g. exports().function("name") etc.

This way we could also have exports().functionType("name") etc. since accessing each exported member type is also a real chore.

The strategy could be to have an inner non-static class as a member to Instance; non-static so that it can easily access the Instance members without having to pass them to its constructor; e.g.

class Instance {

    SomeField f;

   public class Exports() {
        Function function(String name) {
             // `f` is accessible here
        }
   }

}

We might keep Instance#export("name") available both for compatibility and because getting an exported function is a frequent operation.