Mark the initialize functions as deprecated.

Instead, advertise the use of the default Unix generator based on
getrandom/getentropy. This is thread-safe and allows to remove the
mirage-crypto-rng-{lwt,miou-unix,eio,async} packages in the future.

bench/speed.ml

@@ -480,10 +480,12 @@ let benchmarks = [
     throughput_into name (fun dst cs -> DES.ECB.unsafe_encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
 
   bm "fortuna" (fun name ->
-    Mirage_crypto_rng_unix.initialize (module Mirage_crypto_rng.Fortuna);
-    throughput name (fun buf ->
-        let buf = Bytes.unsafe_of_string buf in
-        Mirage_crypto_rng.generate_into buf ~off:0 (Bytes.length buf))) ;
+    begin[@alert "-deprecated"]
+      Mirage_crypto_rng_unix.initialize (module Mirage_crypto_rng.Fortuna);
+      throughput name (fun buf ->
+          let buf = Bytes.unsafe_of_string buf in
+          Mirage_crypto_rng.generate_into buf ~off:0 (Bytes.length buf))
+    end);
 
   bm "getentropy" (fun name ->
     Mirage_crypto_rng_unix.use_getentropy ();

rng/async/mirage_crypto_rng_async.mli

@@ -16,3 +16,4 @@ val initialize
   -> ?sleep:Time_ns.Span.t
   -> 'a Mirage_crypto_rng.generator
   -> unit
+[@@deprecated "Use 'Mirage_crypto_rng_unix.use_default ()' instead."]

rng/eio/mirage_crypto_rng_eio.mli

@@ -39,3 +39,4 @@ val run
   -> 'a Mirage_crypto_rng.generator
   -> _ env
   -> (unit -> 'b) -> 'b
+[@@deprecated "Use 'Mirage_crypto_rng_unix.use_default ()' instead."]

rng/lwt/mirage_crypto_rng_lwt.mli

@@ -9,3 +9,4 @@
     is used to collect entropy.
 *)
 val initialize : ?g:'a -> ?sleep:int64 -> 'a Mirage_crypto_rng.generator -> unit
+[@@deprecated "Use 'Mirage_crypto_rng_unix.use_default ()' instead."]

rng/miou/mirage_crypto_rng_miou_unix.mli

@@ -14,6 +14,7 @@ type rng
 (** Type of tasks seeding the RNG. *)
 
 val initialize : ?g:'a -> ?sleep:int64 -> 'a Mirage_crypto_rng.generator -> rng
+[@@deprecated "Use 'Mirage_crypto_rng_unix.use_default ()' instead."]
 (** [initialize ?g ?sleep (module Generator)] will allow the RNG to operate in a
     returned task. This task periodically launches sub-tasks that seed the
     engine (using [getrandom()], [getentropy()] or [BCryptGenRandom()] depending

rng/unix/mirage_crypto_rng_unix.mli

@@ -8,6 +8,7 @@
 
 (** [initialize ~g rng] will bring the RNG into a working state. *)
 val initialize : ?g:'a -> 'a Mirage_crypto_rng.generator -> unit
+[@@deprecated "Use 'Mirage_crypto_rng_unix.use_default ()' instead."]
 
 (** [getrandom size] returns a buffer of [size] filled with random bytes. *)
 val getrandom : int -> string