improve LOOM_LOG tracing

[hawkw]

This branch makes a few minor improvements to how Loom uses tracing to
generate LOOM_LOG output:

• Added with_test_writer to the tracing subscriber. This allows logs
  written by Loom to be captured by libtest. This way, running Loom
  tests with --show-output will print the LOOM_LOG output from each
  test under that test's heading. Similarly, this allows libtest to
  capture Loom log output by default, and print the output back for
  failed tests.

• Updated to the latest tracing-subscriber version. This gets us
  slightly nicer-looking formatting but is otherwise not that important.

• Added tracing spans for each simulated Loom thread. Now, when we
  switch threads, we also switch to that thread's corresponding
  tracing span. This way, all output in the log is annotated with the
  current thread's name, which should help improve debuggability a bit.

• Added a tracing span with the model's current iteration. This should
  make it much easier to distinguish when a new iteration has started,
  while reading the logs.

Examples

Current master. Note how the log output from all tests is mushed
together, and is not captured by libtest:

This branch. Note how all tracing events are now annotated with
the thread ID and the current iteration, and how libtest is able to
capture output and separate it by which test emitted it:

[carllerche]

🔨