internal: replace TreeSink with a data structure

The general theme of this is to make parser a better independent
library.

The specific thing we do here is replacing callback based TreeSink with
a data structure. That is, rather than calling user-provided tree
construction methods, the parser now spits out a very bare-bones tree,
effectively a log of a DFS traversal.

This makes the parser usable without any *specifc* tree sink, and allows
us to, eg, move tests into this crate.

Now, it's also true that this is a distinction without a difference, as
the old and the new interface are equivalent in expressiveness. Still,
this new thing seems somewhat simpler. But yeah, I admit I don't have a
suuper strong motivation here, just a hunch that this is better.

crates/parser/src/parser.rs

@@ -8,7 +8,6 @@ use limit::Limit;
 use crate::{
     event::Event,
     tokens::Tokens,
-    ParseError,
     SyntaxKind::{self, EOF, ERROR, TOMBSTONE},
     TokenSet, T,
 };
@@ -196,7 +195,7 @@ impl<'t> Parser<'t> {
     /// structured errors with spans and notes, like rustc
     /// does.
     pub(crate) fn error<T: Into<String>>(&mut self, message: T) {
-        let msg = ParseError(Box::new(message.into()));
+        let msg = message.into();
         self.push_event(Event::Error { msg });
     }