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

Errors were being re-propagated as "break".

Also add `error/0` builtin, which made this fix easier.
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.

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
    $
Better quoting in tests/run
Fuzz JSON parser
Streaming parser torture tests