Add ?, .[]?, and .. operators

Make XPath-like `//a/b` recursive structure traversal easier in jq,
which then becomes:

    ..|.a?.b?

The `?` operator suppresses errors about . not being an array or object.
The `..` operator is equivalent to calling the new `recurse_down`
built-in, which in turn is equivalent to

    recurse(.[]?)

Note that `..a` is not supported; neither is `...a`.  That could be add
added, but it doesn't seem worth the trouble of saving the need to type
a '|'.

tests/all.test

@@ -127,6 +127,25 @@ null
 {"foo": {"bar": 20}}
 20
 
+[.[]|.foo?]
+[1,[2],{"foo":3,"bar":4},{},{"foo":5}]
+[3,null,5]
+
+[.[]|.foo?.bar?]
+[1,[2],[],{"foo":3},{"foo":{"bar":4}},{}]
+[4,null]
+
+[..]
+[1,[[2]],{ "a":[1]}]
+[[1,[[2]],{"a":[1]}],1,[[2]],[2],2,{"a":[1]},[1],1]
+
+[.[]|.[]?]
+[1,null,[],[1,[2,[[3]]]],[{}],[{"a":[1,[2]]}]]
+[1,[2,[[3]]],{},{"a":[1,[2]]}]
+
+[.[]|.[1:3]?]
+[1,null,true,false,"abcdef",{},{"a":1,"b":2},[],[1,2,3,4,5],[1,2]]
+[null,"bc",[],[2,3],[2]]
 
 #
 # Multiple outputs, iteration