Zed_lines

`sig
exception Out_of_bounds
type t
val length : Zed_lines.t -> int
val count : Zed_lines.t -> int
val of_rope : Zed_rope.t -> Zed_lines.t
val empty : Zed_lines.t
val line_index : Zed_lines.t -> int -> int
val line_start : Zed_lines.t -> int -> int
val append : Zed_lines.t -> Zed_lines.t -> Zed_lines.t
val insert : Zed_lines.t -> int -> Zed_lines.t -> Zed_lines.t
val remove : Zed_lines.t -> int -> int -> Zed_lines.t
val replace : Zed_lines.t -> int -> int -> Zed_lines.t -> Zed_lines.t
end`