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.

tests/commands_test.rs

@@ -246,13 +246,18 @@ fn it_arg_works_with_many_inputs_to_external_command() {
         let (stdout, stderr) = nu_combined!(
             cwd: dirs.test(), h::pipeline(
             r#"
-                echo file1 file2
+                echo hello world
                 | split-row " "
-                | cat $it
+                | ^echo $it
             "#
         ));
 
-        assert_eq!("text and more text", stdout);
+        #[cfg(windows)]
+        assert_eq!("hello world", stdout);
+
+        #[cfg(not(windows))]
+        assert_eq!("helloworld", stdout);
+
         assert!(!stderr.contains("No such file or directory"));
     })
 }