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
    $

docs/content/3.manual/manual.yml

@@ -103,6 +103,17 @@ sections:
         RS.  This more also parses the output of jq without the `--seq`
         option.
 
+      * `--stream`:
+
+        Parse the input in streaming fashion, outputing arrays of path
+        and leaf values (scalars and empty arrays or empty objects).
+        For example, `"a"` becomes `[[],"a"]`, and `[[],"a",["b"]]`
+        becomes `[[0],[]]`, `[[1],"a"]`, and `[[1,0],"b"]`.
+
+        This is useful for processing very large inputs.  Use this in
+        conjunction with filtering and the `reduce` and `foreach` syntax
+        to reduce large inputs incrementally.
+
       * `--slurp`/`-s`:
 
         Instead of running the filter for each JSON object in the
@@ -2205,6 +2216,29 @@ sections:
             input: '1'
             output: ['[1,2,4,8,16,32,64]']
 
+  - title: 'I/O'
+    body: |
+
+      At this time jq has minimal support for I/O, mostly in the
+      form of control over when inputs are read.  Two builtins functions
+      are provided for this, `input` and `inputs`, that read from the
+      same sources (e.g., `stdin`, files named on the command-line) as
+      jq itself.  These two builtins, and jq's own reading actions, can
+      be interleaved with each other.
+
+      - title: "`input`"
+        body: |
+
+          Outputs one new input.
+
+      - title: "`inputs`"
+        body: |
+
+          Outputs all remaining inputs, one by one.
+
+          This is primarily useful for reductions over a program's
+          inputs.
+
   - title: Assignment
     body: |