Simple and efficient implementation of walk/1 (#2795)

tests/jq.test

@@ -1975,3 +1975,22 @@ implode|explode
 map(try implode catch .)
 [123,["a"],[nan]]
 ["implode input must be an array","string (\"a\") can't be imploded, unicode codepoint needs to be numeric","number (null) can't be imploded, unicode codepoint needs to be numeric"]
+
+# walk
+walk(.)
+{"x":0}
+{"x":0}
+
+walk(1)
+{"x":0}
+1
+
+# The following is a regression test, not a requirement:
+[walk(.,1)]
+{"x":0}
+[{"x":0},1]
+
+# Issue #2584
+walk(select(IN({}, []) | not))
+{"a":1,"b":[]}
+{"a":1}