Merge pull request 'Use Cstruct.length and require cstruct >= 6.0.0' (#56) from cstruct.6.0.0 into main

Reviewed-on: https://git.robur.io/robur/builder-web/pulls/56

builder-web.opam

@@ -17,6 +17,7 @@ build: [
 depends: [
   "builder"
   "dream" {>= "1.0.0~alpha2"}
+  "cstruct" {>= "6.0.0"}
   "bos"
   "hex"
   "caqti"

db/representation.ml

@@ -2,7 +2,7 @@ module Asn = struct
   let decode_strict codec cs =
     match Asn.decode codec cs with
     | Ok (a, cs) ->
-      if Cstruct.len cs = 0
+      if Cstruct.length cs = 0
       then Ok a
       else Error "trailing bytes"
     | Error (`Parse msg) -> Error ("parse error: " ^ msg)