Finish the job of moving shapes into the stream

This commit should finish the `coloring_in_tokens` feature, which moves
the shape accumulator into the token stream. This allows rollbacks of
the token stream to also roll back any shapes that were added.

This commit also adds a much nicer syntax highlighter trace, which shows
all of the paths the highlighter took to arrive at a particular coloring
output. This change is fairly substantial, but really improves the
understandability of the flow. I intend to update the normal parser with
a similar tracing view.

In general, this change also fleshes out the concept of "atomic" token
stream operations.

A good next step would be to try to make the parser more
error-correcting, using the coloring infrastructure. A follow-up step
would involve merging the parser and highlighter shapes themselves.

src/main.rs

@@ -3,9 +3,6 @@ use log::LevelFilter;
 use std::error::Error;
 
 fn main() -> Result<(), Box<dyn Error>> {
-    #[cfg(feature1)]
-    println!("feature1 is enabled");
-
     let matches = App::new("nushell")
         .version(clap::crate_version!())
         .arg(