[discussion] Provide FFI to access ocaml functions from Links code

[frank-emrich]

As we have discussed before, it would be nice to generalize the dynamic loading facilities into something that allows using (somewhat) arbitrary OCaml functions in Links. Since there are a lot of possibilities of approaching this, I would like to collect the design ideas/requirements here.

A few thoughts:

• It seems obvious that OCaml functions should just be invoked from server-only code since we don't have corresponding client-side code.
• I guess that in most use cases, the reason for accessing OCaml functions would be to use code for encryption, hashing, image manipulation, randomness, ...(i.e., first-order code for a particular task that simply would be annoying to re-implemented in Links)
• It would be quite complicated to allow the use of higher-order OCaml functions, but according to the point above, we may be only interested in accessing first-order code in the first place
• I've seen that the dynamic loading of OCaml functions that Harvey implemented this summer (cf. e9be3d7e4b306e2836602fad14c7bcb2ee3f830f) mostly involved the creation of handwritten wrappers for some selected functions in the OCaml standard library, in particular in order to access some data structures like Set, Queue, Stack, Hashtable. If we build something more general, we may be able to use these without handwriting the wrappers. However, for such fundamental data structures it may be more convenient to implement them when needed in the prelude, to avoid fiddling around with foreign functions to use something like a hash table. Alternatively, to avoid re-implementing them, we may decide to add those functions to lib.ml and re-use the corresponding OCaml functions on the server side.

If you can think of other use cases, disagree with some of the above, or have other comments, feel free to let me know.

[jamescheney]

@SimonJF, @slindley, @dhil this is the issue @frank-emrich opened to discuss the "OCaml FFI" design space. Please comment.

[fehrenbach]

We might want to make crypt and verify one of the examples for how to use the FFI and remove them from lib.ml. See also #362.