Modernize external parse and improve trace

The original purpose of this PR was to modernize the external parser to
use the new Shape system.

This commit does include some of that change, but a more important
aspect of this change is an improvement to the expansion trace.

Previous commit 6a7c00ea adding trace infrastructure to the syntax coloring
feature. This commit adds tracing to the expander.

The bulk of that work, in addition to the tree builder logic, was an
overhaul of the formatter traits to make them more general purpose, and
more structured.

Some highlights:

- `ToDebug` was split into two traits (`ToDebug` and `DebugFormat`)
  because implementations needed to become objects, but a convenience
  method on `ToDebug` didn't qualify
- `DebugFormat`'s `fmt_debug` method now takes a `DebugFormatter` rather
  than a standard formatter, and `DebugFormatter` has a new (but still
  limited) facility for structured formatting.
- Implementations of `ExpandSyntax` need to produce output that
  implements `DebugFormat`.

Unlike the highlighter changes, these changes are fairly focused in the
trace output, so these changes aren't behind a flag.

src/parser/hir/binary.rs

@@ -22,8 +22,8 @@ impl fmt::Display for Binary {
     }
 }
 
-impl ToDebug for Binary {
-    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+impl FormatDebug for Binary {
+    fn fmt_debug(&self, f: &mut DebugFormatter, source: &str) -> fmt::Result {
         write!(f, "{}", self.left.debug(source))?;
         write!(f, " {} ", self.op.debug(source))?;
         write!(f, "{}", self.right.debug(source))?;