Pluggable Renderer design

[bdarcus]

It should be trivial to configure different output renderers (HTML, RTF, ODT, LaTeX, etc.), and to create new ones (like Typst). I'm a bit uncertain on the ideal design ATM, however.

1. pluggable output renderers, starting with basic string, and maybe Djot integration #86
2. borrow ideas from https://github.com/obsidiandynamics/stanza; it has a markdown renderer, for example

Here's how one uses jotdown, which has a Render trait to offer configurable output.

use jotdown::Render;
let djot_input = "hello *world*!";
let events = jotdown::Parser::new(djot_input);
let mut html = String::new();
jotdown::html::Renderer::default().push(events, &mut html);
assert_eq!(html, "<p>hello <strong>world</strong>!</p>\n");

While that's an event-based processor (hence the push method), and this isn't, here's how they define that trait:

pub trait Render {
    /// Push owned [`Event`]s to a unicode-accepting buffer or stream.
    fn push<'s, I, W>(&self, events: I, out: W) -> fmt::Result
    where
        I: Iterator<Item = Event<'s>>,
        W: fmt::Write;

    /// Write owned [`Event`]s to a byte sink, encoded as UTF-8.
    ///
    /// NOTE: This performs many small writes, so IO writes should be buffered with e.g.
    /// [`std::io::BufWriter`].
    fn write<'s, I, W>(&self, events: I, out: W) -> io::Result<()>
    where
        I: Iterator<Item = Event<'s>>,
        W: io::Write,
    {
        let mut out = WriteAdapter {
            inner: out,
            error: Ok(()),
        };

        self.push(events, &mut out).map_err(|_| match out.error {
            Err(e) => e,
            _ => io::Error::new(io::ErrorKind::Other, "formatter error"),
        })
    }

Here's the stanza Renderer trait, and it's markdown implementation:

pub trait Renderer {
    type Output: Display;

    #[inline]
    fn render(&self, table: &Table) -> Self::Output {
        self.render_with_hints(table, &[])
    }

    fn render_with_hints(&self, table: &Table, hints: &[RenderHint]) -> Self::Output;
}