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
    $

builtin.c

@@ -862,6 +862,18 @@ static jv f_modulemeta(jq_state *jq, jv a) {
   return load_module_meta(jq, a);
 }
 
+static jv f_input(jq_state *jq, jv input) {
+  jv_free(input);
+  jq_input_cb cb;
+  void *data;
+  jq_get_input_cb(jq, &cb, &data);
+  jv v = cb(jq, data);
+  if (jv_is_valid(v) || jv_invalid_has_msg(jv_copy(v)))
+    return v;
+  return jv_invalid_with_msg(jv_string("break"));
+}
+
+
 
 #define LIBM_DD(name) \
   {(cfunction_ptr)f_ ## name, "_" #name, 1},
@@ -912,6 +924,7 @@ static const struct cfunction function_list[] = {
   {(cfunction_ptr)f_env, "env", 1},
   {(cfunction_ptr)f_match, "_match_impl", 4},
   {(cfunction_ptr)f_modulemeta, "modulemeta", 1},
+  {(cfunction_ptr)f_input, "_input", 1},
 };
 #undef LIBM_DD
 
@@ -1014,6 +1027,7 @@ static const char* const jq_builtins[] = {
   "def nulls: select(type == \"null\");",
   "def values: select(. != null);",
   "def scalars: select(. == null or . == true or . == false or type == \"number\" or type == \"string\");",
+  "def scalars_or_empty: select(. == null or . == true or . == false or type == \"number\" or type == \"string\" or ((type==\"array\" or type==\"object\") and length==0));",
   "def leaf_paths: paths(scalars);",
   "def join($x): reduce .[] as $i (\"\"; . + (if . == \"\" then $i else $x + $i end));",
   "def flatten: reduce .[] as $i ([]; if $i | type == \"array\" then . + ($i | flatten) else . + [$i] end);",