Collect runtime profiling information in CTF format

This library can be used to trace execution of OCaml/Lwt programs (such as Mirage unikernels) at the level of Lwt threads. The traces can be viewed using JavaScript or GTK viewers provided by [mirage-trace-viewer][] or processed by tools supporting the [Common Trace Format (CTF)][ctf]. Some example traces can be found in the blog post Visualising an Asynchronous Monad.

Libraries can use the functions mirage-profile provides to annotate the traces with extra information. When compiled against a normal version of Lwt, mirage-profile's functions are null-ops (or call the underlying untraced operation, as appropriate) and OCaml's cross-module inlining will optimise these calls away, meaning there should be no overhead in the non-profiling case.