More general cstring, chose delimiter

[roblabla]

It'd be nice to have a character (or substr)-delimited string type, kind of like cstring but where we can chose the delimiter. For instance, the HTTP protocol (and most other text protocols) are effectively just

{
  "types": {
    "string": ["dstring", { "delimiter": "\n" }]
  },
  "packet": "string"
}

(OK OK, this is a very ugly protocol definition, but it works :P)