Add the shift function on Bstr

lib/cachet.ml

@@ -391,6 +391,11 @@ module Bstr = struct
     match rev with
     | true -> rdrop ?min ?max ?sat bstr
     | false -> fdrop ?min ?max ?sat bstr
+
+  let shift bstr off =
+    if off > length bstr then invalid_arg "Cachet.Bstr.shift";
+    let len = length bstr - off in
+    Bigarray.Array1.sub bstr off len
 end
 
 external hash : (int32[@unboxed]) -> int -> (int32[@unboxed])

lib/cachet.mli

@@ -187,6 +187,9 @@ module Bstr : sig
       {[
         (if rev then fst else snd) @@ span ~rev ~min ~max ~sat bstr
       ]} *)
+
+  val shift : t -> int -> t
+  (** [shift bstr n] is [sub bstr n (length bstr - n)]. *)
 end
 
 type slice = private { offset: int; length: int; payload: Bstr.t }