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
    $

lexer.l

@@ -100,7 +100,7 @@ struct lexer_param;
   }
   (\\[^u(]|\\u[a-zA-Z0-9]{0,4})+ {
     /* pass escapes to the json parser */
-    jv escapes = jv_string_fmt("\"%.*s\"", yyleng, yytext);
+    jv escapes = jv_string_fmt("\"%.*s\"", (int)yyleng, yytext);
     yylval->literal = jv_parse_sized(jv_string_value(escapes), jv_string_length_bytes(jv_copy(escapes)));
     jv_free(escapes);
     return QQSTRING_TEXT;