Replace dynlink method by a 2-stage build

[TheLortex]

Hi, this is quite a heavy PR. I can try to minimize the diff if you want.

Why

The dynlink method to load config.ml feels quite fragile, as it requires mirage and config.ml to be built with the same code / options. When compiling using dune, these assumptions can be broken:

• config.ml can be built with a local instance of the mirage source code.
• Link-time optimizations might not be enabled for mirage but are enabled for config.ml.

Proposal

Instead of dynlinking config.ml, we can perform a 2-stage build of the configuration ! (1) Locally build config.ml (2) Execute config.ml, which calls some entry function in Functoria_app

There are several ways of doing it:

• Breaking changes for both config.ml and mirage:
  • Separate Functoria_app.Make in two functors, one that builds the config.ml and one that is called by the built config.ml
  • Change register ... to a more appropriate name
• Breaking changes for mirage:
  • Only perform the separation of Make and have register ... as the entry point of (2)
• No breaking changes:
  • Keep everything under Make and hack with the names that are currently used.

I've implemented the intermediate solution. This is probably far from perfect but I'd like to have feedback on that :)

[cfcs]

@TheLortex I'm super excited about this since I think it can pave the way for external device types/implementations. I cannot get this to build though, can you link to the corresponding mirage branch?

[TheLortex]

The PR currently breaks API with Mirage, but it's not necessary, I'm waiting for comments on this.

There are two branches of https://github.com/TheLortex/mirage that implement the change of this PR:

• functoria-fix (https://github.com/TheLortex/mirage/tree/functoria-fix):
  • builds unikernels using ocamlbuild, should be stable to test this PR
• full-dune+functoria-fix (https://github.com/TheLortex/mirage/tree/full-dune+functoria-fix):
  • new shiny dune build relying on dune 1.10 so that's mostly experimental

[samoht]

Looks good, merging!