lang: parse, core: world: Add a collect package

This lets us look at the available resource data for collection, and to
filter it so we can decide what we want to collect on our machine.

Other types of collect functions could be added in the future.

examples/lang/collect0.mcl

@@ -0,0 +1,52 @@
+import "fmt"
+import "golang/strings" as golang_strings
+import "iter"
+import "world/collect"
+
+file "/tmp/foo" {
+	state => $const.res.file.state.exists,
+	content => "i am foo\n",
+
+	Meta:export => ["*",],
+	#Meta:export => ["${hostname}",], # alternatively
+}
+file "/tmp/fah" {
+	state => $const.res.file.state.exists,
+	content => "i am fah\n",
+
+	#Meta:export => ["*",],
+	Meta:export => ["${hostname}",],
+}
+
+$all = collect.res("file") # []struct{name str; host str;}
+
+# any kind of filter code that you want!
+$fn = func($st) {
+	golang_strings.has_prefix($st->name, "/tmp/fo")
+}
+
+$filtered = iter.filter($all, $fn)
+
+print "all" {
+	msg => fmt.printf("%v", $all),
+
+	Meta:autogroup => false,
+}
+print "filtered" {
+	msg => fmt.printf("%v", $filtered),
+
+	Meta:autogroup => false,
+}
+
+collect file $filtered {
+	Meta:hidden => true, # let's not apply these for now
+}
+
+# you can do it manually like this
+#$collected = [
+#	struct{name => "/tmp/foo", host => "${hostname}",},
+#]
+#
+#collect file $collected {
+#	Meta:hidden => false,
+#}