Generic ast (#1829)

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

pkg/yqlib/doc/usage/convert.md

@@ -14,7 +14,7 @@ Given a sample.json file of:
 ```
 then
 ```bash
-yq -P '.' sample.json
+yq -p=json sample.json
 ```
 will output
 ```yaml
@@ -30,16 +30,16 @@ Given a sample.json file of:
 ```
 then
 ```bash
-yq -P '.' sample.json
+yq -p=json sample.json
 ```
 will output
 ```yaml
 a: Easy! as one two three
 b:
-  c: 2
-  d:
-    - 3
-    - 4
+    c: 2
+    d:
+        - 3
+        - 4
 ```
 
 ## Encode json: simple

pkg/yqlib/doc/usage/xml.md

@@ -129,15 +129,13 @@ zoo:
 ```
 
 ## Parse xml: force all as an array
-Because of the way yq works, when updating everything you need to update the children before the parents. By default `..` will match parents first, so we reverse that before updating.
-
 Given a sample.xml file of:
 ```xml
 <zoo><thing><frog>boing</frog></thing></zoo>
 ```
 then
 ```bash
-yq -oy '([..] | reverse | .[]) |= [] + .' sample.xml
+yq -oy '.. |= [] + .' sample.xml
 ```
 will output
 ```yaml