refactor: use external crate for displaying tracing spans

[matklad]

When debugging #4771, I had to copy-paste tracing_timings module to get
a useful profiling output, which isn't ideal. So I improved and
published that code to crates.io as tracing-span-tree.

In particular, this new implementation prints the trace in the correct
order, and it also has an .aggregate(true) mode for cases where you
run the profiled code in a loop and want a summary.

Example without aggregation:

  328.25ms         vm_script_smoke_test
    306.34ms         run_vm
      306.32ms         run_wasmer
        37.16µs          get_key
        301.50ms         compile_and_serialize_wasmer
        4.44ms           run_method
          2.53ms           run_method/instantiate
          128.17µs         run_method/call
          1.74ms           run_method/drop_instance
    4.50ms           run_vm
      4.49ms           run_wasmer
        31.81µs          get_key
        4.25ms           run_method
          2.45ms           run_method/instantiate
          45.46µs          run_method/call
          1.72ms           run_method/drop_instance
    4.43ms           run_vm
      4.42ms           run_wasmer
        22.43µs          get_key
        4.21ms           run_method
          2.41ms           run_method/instantiate
          42.84µs          run_method/call
          1.72ms           run_method/drop_instance
    4.81ms           run_vm
      4.80ms           run_wasmer
        22.09µs          get_key
        4.58ms           run_method
          2.40ms           run_method/instantiate
          419.57µs         run_method/call
          1.73ms           run_method/drop_instance

Example with aggregation (unit-less number is the number of aggregated calls):

  333.98ms         vm_script_smoke_test
    325.54ms  4      run_vm
      325.48ms  4      run_wasmer
        307.07ms         compile_and_serialize_wasmer
        129.19µs  4      get_key
        17.32ms   4      run_method
          612.46µs  4      run_method/call
          6.63ms    4      run_method/drop_instance
          9.94ms    4      run_method/instantiate

Test Plan

This is a debugging utility which is not enabled in production, so I just manually verified that the output makes sense.

[ailisp]

Looks good! Could you paste a sample output in PR description to demonstrate?

[matklad]

Good call!