Allow zero-length buffers in jv_parser_set_buf()

If the caller is at EOF and has no more bytes to feed the parser, how is
the parser to be told about the EOF condition?  A small fix to allow
zero-length buffers in jv_parser_set_buf() fixes this problem (it also
makes it easier to deal with async I/O: feed the parser whatever is
available, including nothing).
Fix `foreach` non-progation of errors

Also add `error/0` builtin.
Add BLOCK_8() macro
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