lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/core/lookup.go

@@ -179,6 +179,12 @@ func (obj *LookupFunc) Build(typ *types.Type) (*types.Type, error) {
 		// programming error
 		return nil, err
 	}
+
+	if _, ok := f.(interfaces.CallableFunc); !ok {
+		// programming error
+		return nil, fmt.Errorf("not a CallableFunc")
+	}
+
 	bf, ok := f.(interfaces.BuildableFunc)
 	if !ok {
 		// programming error
@@ -230,3 +236,13 @@ func (obj *LookupFunc) Stream(ctx context.Context) error {
 	}
 	return obj.fn.Stream(ctx)
 }
+
+// Call returns the result of this function.
+func (obj *LookupFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	cf, ok := obj.fn.(interfaces.CallableFunc)
+	if !ok {
+		// programming error
+		return nil, fmt.Errorf("not a CallableFunc")
+	}
+	return cf.Call(ctx, args)
+}