builtin: add pick(stream)

pick(stream) works on both arrays and objects

Restrictions on the dot-paths are specified in manual.yml

See #2578

tests/man.test

@@ -219,6 +219,14 @@ map_values(.+1)
 {"a": 1, "b": 2, "c": 3}
 {"a": 2, "b": 3, "c": 4}
 
+pick(.a, .b.c, .x)
+{"a": 1, "b": {"c": 2, "d": 3}, "e": 4}
+{"a":1,"b":{"c":2},"x":null}
+
+[1,2,3,4] | pick(.[2], .[0], .[0])
+[1,2,3,4]
+[1,null,3]
+
 path(.a[0].b)
 null
 ["a",0,"b"]