Algorithm for parsing nested links, images, emphasis, and quotes
================================================================

When we're parsing inlines and we hit

- a run of `*` or `_` characters
- a `[` or `![`

we insert a text node with the literal content, and add a pointer
to this text node to the **delimiter stack.**

The **delimiter stack** is a doubly linked list.  Each
element contains a pointer to a text node, plus information about

- the type of delimiter (`[`, `![`, `*`, `_`)
- the number of delimiters,
- whether the delimiter is "active" (all are active to start), and
- whether the delimiter is a potential opener, a potential closer,
  or both.

When we hit a `]` character, we call the `look_for_link_or_image`
procedure (see below).

When we hit the end of the input, we call the `process_emphasis`
procedure (see below), with `stack_bottom` = NULL.

`look_for_link_or_image`
------------------------

Starting at the top of the delimiter stack, we look backwards
through the stack for a `[` or `![` delimiter.

If we don't find one, we return a literal text node `]`.

If we do find one, but it's not *active*, we remove the inactive
delimiter from the stack, and return a literal text node `]`.

If we find one and it's active, then we parse ahead to see if
we have an inline link/image, reference link/image, compact reference
link/image, or shortcut reference link/image.

If we don't, then we remove the `[` or `![` delimiter from the
delimiter stack and return a literal text node `]`.

If we do, then

- We return a link or image node whose children are the inlines
  after the text node pointed to by the opening delimiter.

- We run `process_emphasis` on these inlines, with the `[` opener
  as `stack_bottom`.

- We remove the opening delimiter.

- If we have a link (and not an image), we also set all
  `[` delimiters before the opening delimiter to *inactive*.  (This
  will prevent us from getting links within links.)


`process_emphasis`
------------------

Parameter `stack_bottom` sets a lower bound to how far we
descend in the **delimiter stack**.  If it is NULL, we can
go all the way to the bottom.  Otherwise, we stop before
visiting `stack_bottom`.

Let `current_position` point to the element on the delimiter
just above `stack_bottom` (or the first element if `stack_bottom`
is NULL).

We keep track of the `openers_bottom` for each delimiter
type (`*`, `_`).  Initialize this to `stack_bottom`.

Then we repeat the following until we run out of potential
closers:

- Move `current_position` forward in the delimiter stack (if needed)
  until we find the first potential closer with delimiter `*` or `_`.
  (This will be the potential closer closest
  to the beginning of the input -- the first one in parse order.)

- Now, look back in the stack (staying above `stack_bottom` and
  the `openers_bottom` for this delimiter type) for the
  first matching potential opener ("matching" means same delimiter).

- If one is found:

  - Figure out whether we have emphasis or strong emphasis:
    if both closer and opener spans have length >= 2, we have
    strong, otherwise regular.
  - Insert an emph or strong emph node accordingly, after
    the text node corresponding to the opener.
  - Remove delimiters between opener and closer from the delimiter
    stack.
  - Remove 1 (for regular emph) or 2 (for strong emph) delimiters
    from the opening and closing text nodes.  If they become empty
    as a result, remove them and remove the corresponding element
    of the delimiter stack.  If the closing node is removed, reset
    `current_position` to the next element in the stack.

- If none in found:

  - Set `openers_bottom` to the element before `current_position`.
    (We know that there are no openers for this kind of closer up to and
    including this point, so this puts a lower bound on future searches.)
  - If the closer at `current_position` is not a potential opener,
    remove it from the delimiter stack (since we know it can't
    be a closer either).
  - Advance `current_position` to the next element in the stack.

- Repeat.

- After we're done, remove all delimiters above `stack_bottom` from the
  delimiter stack.