4987 - support `browse_trace` tool in SubX

I've extracted it into a separate binary, independent of my Mu prototype.

I also cleaned up my tracing layer to be a little nicer. Major improvements:

- Realized that incremental tracing really ought to be the default.
  And to minimize printing traces to screen.

- Finally figured out how to combine layers and call stack frames in a
  single dimension of depth. The answer: optimize for the experience of
  `browse_trace`. Instructions occupy a range of depths based on their call
  stack frame, and minor details of an instruction lie one level deeper
  in each case.

Other than that, I spent some time adjusting levels everywhere to make
`browse_trace` useful.

1 files changed, 13 insertions, 1 deletions

diff --git a/Readme.md b/Readme.md
index 54af1049..f1dd1be6 100644
--- a/Readme.md
+++ b/Readme.md
@@ -403,7 +403,19 @@ c) Try running the tests:
 d) Check out [the programming environment](https://github.com/akkartik/mu/tree/master/edit#readme),
 the largest app built so far in Mu.
 
-e) Look at the `build` scripts. Mu's compilation process is itself designed to
+e) Check out the tracing infrastructure which gives you a maps-like zoomable
+UI for browsing Mu's traces:
+
+  ```shell
+  $ ./mu --trace nqueens.mu  # just an example
+  saving trace to 'last_run'
+  $ ./browse_trace/browse_trace last_run
+  # hit 'q' to exit
+  ```
+
+For more details see the [Readme](browse_trace/Readme.md).
+
+f) Look at the `build` scripts. Mu's compilation process is itself designed to
 support staged learning. Each of the scripts (`build0`, `build1`, `build2`,
 etc.) is self-contained and can compile the project by itself. Successive
 versions add new features and configurability -- and complexity -- to the