Fix streaming bug (fuzzing)
Allow resetting of jq err callback
Add `label $name | EXP`; fix `break`

This is to fix the problem where `break` is dynamic, not lexical.

With this it should be possible to do this sort of thing:

    label $break | inputs | if ... then $break|error else . end

This is a backwards-incompatible change for master, but the previous
`break` hadn't shipped yet.

Still needed:

 - testing
Add support for testing erroneous programs

    $ jq --run-tests
    %%FAIL
    break
    jq: error: break used outside labeled control structure
    ^D
    1 of 1 tests passed (0 malformed)
    $
Test labels