engine, lang: Modern exported resources

I've been waiting to write this patch for a long time. I firmly believe
that the idea of "exported resources" was truly a brilliant one, but
which was never even properly understood by its original inventors! This
patch set aims to show how it should have been done.

The main differences are:

* Real-time modelling, since "once per run" makes no sense.
* Filter with code/functions not language syntax.
* Directed exporting to limit the intended recipients.

The next step is to add more "World" reading and filtering functions to
make it easy and expressive to make your selection of resources to
collect!

docs/resource-guide.md

@@ -523,9 +523,10 @@ graph edges from another resource. These values are consumed during the
 any resource that has an appropriate value and that has the `Sendable` trait.
 You can read more about this in the Send/Recv section below.
 
-### Collectable
+### Exportable
 
-This is currently a stub and will be updated once the DSL is further along.
+Exportable allows a resource to tell the exporter what subset of its data it
+wishes to export when that occurs. It is rare that you will need to use this.
 
 ## Resource Initialization