lang: funcs: facts: Add a separate callable interface

Add some symmetry to our interfaces for now, even though I'd love to
drop the idea of "facts" altogether.

lang/funcs/facts/facts.go

@@ -106,7 +106,14 @@ type Fact interface {
 	Info() *Info
 	Init(*Init) error
 	Stream(context.Context) error
-
+}
-	// TODO: should we require this here? What about a CallableFact instead?
+
-	Call(context.Context) (types.Value, error)
+// CallableFact is a function that takes no args, and that can be called
+// statically if we want to do it speculatively or from a resource.
+type CallableFact interface {
+	Fact // implement everything in Fact but add the additional requirements
+
+	// Call this fact and return the value if it is possible to do so at
+	// this time.
+	Call(ctx context.Context) (types.Value, error)
 }