Full tracing support

[talex5]

#635 added initial support for tracing using OCaml's runtime events system. However, more features are needed to make it useful:

• Add events about cancellation contexts (Improve tracing #656).
• Add more labels to make the traces clearer (see Tracing: add labels to switches #661).
• Trace syscalls.
• Trace systheads.
• Write a visualisation tool that can take advantage of the extra structure in Eio (https://github.com/ocaml-multicore/eio-trace).