Generic ast (#1829)

Remove dependency on yaml.Node for internal AST representation. Yaml decoder is now just another decoder.

pkg/yqlib/operator_collect_test.go

@@ -5,6 +5,13 @@ import (
 )
 
 var collectOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		expression: `["x", "y"] | .[1]`,
+		expected: []string{
+			"D0, P[1], (!!str)::y\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   ``,