paths now includes paths to nulls

There are a few reasonable ways recurse could behave. The standard
libraries implemntation does the following:

`recurse(f;cond)` yields ., then . | f if . | f satisfies cond, then
                                 . | f | f if . | f | f satisfies cond,
                                 etc...

But it turns out that for paths, you want the following behavior:
`recurse(f;cond)` yields ., then . | f is . satisfies cond, then
                                 . | f | f if . | f satisfies cond,
                                 etc.

The difference is non-trivial (and there are actually a few degrees of
freedom here), so I gave paths its own definition, rather than adding a
new builtin. This means we don't have two builtin recurses floating
around.

src/builtin.jq

@@ -29,7 +29,8 @@ def indices($i): if type == "array" and ($i|type) == "array" then .[$i]
   else .[$i] end;
 def index($i):   indices($i) | .[0];       # TODO: optimize
 def rindex($i):  indices($i) | .[-1:][0];  # TODO: optimize
-def paths: path(recurse(if (type|. == "array" or . == "object") then .[] else empty end))|select(length > 0);
+def paths: def recurse_pre(f; cond): def r: ., (select(cond) | f | r); r;
+           path(recurse_pre(.[]?;.!=null))|select(length > 0);
 def paths(node_filter): . as $dot|paths|select(. as $p|$dot|getpath($p)|node_filter);
 def any(generator; condition):
         [label $out | foreach generator as $i

tests/jq.test

@@ -743,6 +743,16 @@ path(.a[path(.b)[0]])
 [1,[[],{"a":2}]]
 [[0],[1],[1,0],[1,1],[1,1,"a"]]
 
+# Make sure we get paths to nulls
+[paths]
+[1,null]
+[[0],[1]]
+
+# Make sure we don't get paths to lone items
+[paths]
+5
+[]
+
 [leaf_paths]
 [1,[[],{"a":2}]]
 [[0],[1,1,"a"]]