lang: funcs, parser: Add improved panic magic

This is a newer implementation of the panic magic. I kept the old commit
in for posterity and to show the difference. The two versions are
identical to the end-user with one exception: the newer version doesn't
include a useless panic resource in the graph when there is no panic. In
this version, the panic function returns false and the if statement it's
the condition of, doesn't produce the resource within. On error, we
still consume the function in the if expression, and doing so causes
everything to shutdown.

The other benefit is that the implementation is much cleaner and doesn't
need the interpolate hack.

lang/funcs/core/panic_func.go

@@ -26,21 +26,18 @@ import (
 
 func init() {
 	simple.Register("panic", &types.FuncValue{
-		T: types.NewType("func(x str) str"),
+		T: types.NewType("func(x str) bool"),
 		V: Panic,
 	})
 }
 
 // Panic returns an error when it receives a non-empty string. The error should
-// cause the function engine to shutdown.
+// cause the function engine to shutdown. If there's no error, it returns false.
 func Panic(input []types.Value) (types.Value, error) {
 	if s := input[0].Str(); s != "" {
-		// This StrValue not really used here, since we error...
-		return &types.StrValue{
-			V: s,
-		}, fmt.Errorf("panic occurred: %s", s)
+		return nil, fmt.Errorf("panic occurred: %s", s)
 	}
-	return &types.StrValue{
-		V: "panic", // name can't be empty
+	return &types.BoolValue{
+		V: false,
 	}, nil
 }