lang: ast, interfaces, interpret: Change the Output sig

This plumbs through the new Output method signature that accepts a table
of function pointers to values and relies on the previous storing of the
function pointers to be used for the lookup right now. This has the
elegant side-effect that Output generation could run in parallel with
the graph engine, as the engine only needs to pause to take a snapshot
of the current values tables.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/interfaces/ast.go

@@ -76,8 +76,9 @@ type Stmt interface {
 	Graph() (*pgraph.Graph, error)
 
 	// Output returns the output that this "program" produces. This output
-	// is what is used to build the output graph.
-	Output() (*Output, error)
+	// is what is used to build the output graph. It requires the input
+	// table of values that are used to populate each function.
+	Output(map[Func]types.Value) (*Output, error)
 }
 
 // Expr represents an expression in the language. Expr implementations must have