Add Streaming parser (--stream)

Streaming means that outputs are produced as soon as possible.  With the
`foreach` syntax one can write programs which reduce portions of the
streaming parse of a large input (reduce into proper JSON values, for
example), and discard the rest, processing incrementally.

This:

    $ jq -c --stream .

should produce the same output as this:

    $ jq -c '. as $dot | path(..) as $p | $dot | getpath($p) | [$p,.]'

The output of `jq --stream .` should be a sequence of`[[<path>],<leaf>]`
and `[[<path>]]` values.  The latter indicate that the array/object at
that path ended.

Scalars and empty arrays and objects are leaf values for this purpose.

For example, a truncated input produces a path as soon as possible, then
later the error:

    $ printf '[0,\n'|./jq -c --stream .
    [[0],0]
    parse error: Unfinished JSON term at EOF at line 3, column 0
    $

jq.h

@@ -22,6 +22,10 @@ void jq_start(jq_state *, jv value, int);
 jv jq_next(jq_state *);
 void jq_teardown(jq_state **);
 
+typedef jv (*jq_input_cb)(jq_state *, void *);
+void jq_set_input_cb(jq_state *, jq_input_cb, void *);
+void jq_get_input_cb(jq_state *, jq_input_cb *, void **);
+
 void jq_set_attrs(jq_state *, jv);
 jv jq_get_attrs(jq_state *);
 jv jq_get_jq_origin(jq_state *);