Try to use `fiber` support to implement proper support for non-hierarchical tracing spans

[nagisa]

Tracing spans don't need to be hierarchical, it is entirely possible to have spans looking like this

| -- S1 -- |
   | -- S2 -- |
      | -- S3 -- |

Tracy itself requires however that all spans entered in a single thread share a strong hierarchical relationship and requires that S2 and S3 both terminate before S1.

Tracy recently introduced support for fibers which, to the best of my knowledge, should give enough control to properly represent non-hierarchical spans like these.

[nagisa]

A downside to this is a significant increase in instrumentation overhead when the fibers feature is enabled.