Implement personalisation for Blake2

[dannywillems]

I forked the Blake2 OCaml implementation to add the personalisation, with some tests from Zcash, see https://gitlab.com/dannywillems/ocaml-ec/-/merge_requests/18, directory src/digestif.
I am happy to make an MR, but a discussion for the interface is necessary. The fork in ocaml-ec changes it slightly and only uses blake2.

[dinosaure]

Which parameters you want to personalize in BLAKE2? The interface currently allows to personalize the length of the hash:

digestif/src/digestif.mli

Lines 232 to 238 in 8919076

	module Make_BLAKE2B (D : sig
	val digest_size : int
	end) : S
	module Make_BLAKE2S (D : sig
	val digest_size : int
	end) : S

I'm aware about some others parameters but I need to know which one you want to change. digestif shares the same interface for the C implementation and the OCaml implementation, we must implement what you want for both. This is the first constraint, the second constraint is the common interface module type S for any hashes implementations.

From what I see, you want to add a new argument for the init function. I would like to say that the best is to provide a special new function init_with_iv or something like that and provide it here:

digestif/src/digestif.mli

Lines 218 to 228 in 8919076

	module BLAKE2B : sig
	include S
	module Keyed : MAC with type t = t
	end
	module BLAKE2S : sig
	include S
	module Keyed : MAC with type t = t
	end

[dannywillems]

The personalisation I implemented is the parameter personal, see https://github.com/mirage/digestif/blob/master/src-ocaml/baijiu_blake2s.ml#L88. It might be better to allow also adding a salt (see https://github.com/mirage/digestif/blob/master/src-ocaml/baijiu_blake2s.ml#L87).
I have just finished splitting the fork of Digestif I used. The commit you might be interested in is https://gitlab.com/dannywillems/ocaml-ec/-/merge_requests/18/diffs?commit_id=389f457d567de58fccbea738a5259d164ebcf80f.
(it might be a bit ugly, but as it is not going to be exposed in the library ocaml-ec, that's fine).

[dinosaure]

As I said, an other init function with personalization should be fine for me 👍 . So if you want to propose something like that, I will happy to review and merge.