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
    $

tests/torture/input0.json

@@ -0,0 +1,7 @@
+0 12 true false null [] {} [{}] [[]]
+[0,01,[12,22,[34,[45,56],7]],[]]
+{"a":[1]}
+{"a":[{}]}
+{"a":[{},[],[[]]]}
+{"a":[{"b":[]}]}
+{"a":[{"b":[]},{},[2]]}