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/documentation.md

@@ -297,6 +297,49 @@ This meta param is a safety measure to make your life easier. It works for all
 resources. If someone comes up with a resource which would routinely start with
 a dollar sign, then we can revisit the default for this resource kind.
 
+#### Hidden
+
+Boolean. Hidden means that this resource will not get executed on the resource
+graph on which it is defined. This can be used as a simple boolean switch, or,
+more commonly in combination with the Export meta param which specifies that the
+resource params are exported into the shared database. When this is true, it
+does not prevent export. In fact, it is commonly used in combination with
+Export. Using this option will still include it in the resource graph, but it
+will exist there in a special "mode" where it will not conflict with any other
+identically named resources. It can even be used as part of an edge or via a
+send/recv receiver. It can NOT be a sending vertex. These properties
+differentiate the use of this instead of simply wrapping a resource in an "if"
+statement.
+
+#### Export
+
+List of strings. Export is a list of hostnames (and/or the special "*" entry)
+which if set, will mark this resource data as intended for export to those
+hosts. This does not prevent any users of the shared data storage from reading
+these values, so if you want to guarantee secrecy, use the encryption
+primitives. This only labels the data accordingly, so that other hosts can know
+what data is available for them to collect. The (kind, name, host) export triple
+must be unique from any given exporter. In other words, you may not export two
+different instances of a kind+name to the same host, the exports must not
+conflict. On resource collect, this parameter is not preserved.
+
+```mcl
+file "/tmp/foo" {
+	state => "exists",
+	content => "i'm exported!\n",
+
+	Meta:hidden => true,
+	Meta:export => ["h1",],
+}
+
+file "/tmp/foo" {
+	state => "exists",
+	content => "i'm exported AND i'm used here\n",
+
+	Meta:export => ["h1",],
+}
+```
+
 #### Reverse
 
 Boolean. Reverse is a property that some resources can implement that specifies