Allow C-coded functions to `empty`

Just return a jv_invalid() without a message.
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.  We should add
an exit builtin (or use `break` or `error`) and processing can even
terminate early.

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 `[[<path>],<leaf>]`.  To prevent
ambiguities, the start of any array and object yields a path with a -1
or null (respectively) for the last component of the path, and no leaf.

Hmmm: The -1/null thing is a bit bogus.  Instead we should return empty
arrays and objects as leaves!

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

    $ printf '[0,\n'|./jq -c --stream .
    [[-1]]
    parse error: Unfinished JSON term
    $

Here's an example without that error:

    $ ./jq -c --stream .
      0 [1] [2,3] [4,[5,[6,7]]]
      [[],0]
      [[-1]]
      [[0],1]
      [[-1]]
      [[0],2]
      [[1],3]
      [[-1]]
      [[0],4]
      [[1,-1]]
      [[1,0],5]
      [[1,1,-1]]
      [[1,1,0],6]
      [[1,1,1],7]
    $

Anyways, a work in progress.

Testing will involve torture JSON inputs, and their hand-coded streamed
equivalents for comparison.

Fix streaming bugs and add tests
Always stream object/array endings

It turns out that we must always do this to avoid ambiguities.  E.g., an
input sequence like `{"a":0} {"b":1}` would be indistinguishable from
`{"a":0,"b":1}` otherwise.

Also, indicating the ends of non-empty arrays/objects (empty ones are
"leaves") helps consumers output as soon as a reduction is ready.

We now have two output forms when streaming:

    [<path>, <leaf>]
    [<path>]

The second occurs whenever a non-empty array or object is closed.