TIME, CLOCK, RANDOM - use the linking trick instead of functors

[Hannes Mehnert]

Dear everyone,

as discussed on the retreat last year, and in May 2024 on this list, and 
further in the Mirage issue tracker, finally I went through the quite 
large corpus of packages to provide a showcase how this will look like 
(including working mirage-skeleton unikernels).

So, if you haven't followed the discussion, the idea is to use the 
linking trick (where we use an OCaml interface for compilation, and when 
linking we provide the "right" implementation) -- so our libraries do 
not need to be concerned about the actual implementation. We use this 
for defining the interfaces for Mirage_sleep, Mirage_mtime, 
Mirage_ptime, and also for Mirage_crypto_rng -- previously we already 
applied this for Mirage_bootvar.

The result is that we no longer need to functorize over the 
TIME/PCLOCK/MCLOCK/RANDOM "devices" in each unikernel.

Et voila, take a deep breath and look at the diff (ignoring whitespaces) 
https://github.com/mirage/mirage-skeleton/pull/407/files?w=1 of our 
mirage-skeleton example unikernel repository.

More details are in https://github.com/mirage/mirage/pull/1599, and even 
more in the individual packages 
https://github.com/hannesm/mirage-time-defunctorised (also 
https://github.com/mirage/mirage-sleep 
https://github.com/mirage/mirage-mtime 
https://github.com/mirage/mirage-ptime).

It would be lovely to hear your thoughts on this. If it is positive, I'm 
keen to open PRs and cut releases while the code is still hot, since 
otherwise we may suffer from bitrot again.


Best,

Hannes