enhance: update inline link type

[RCmerci]

• update Tag of string -> Tag of Inline.t list
• remove inline.Block_reference s type , which is moved to inline.Link with inline.url=Block_ref <id>
• update inline.url type

  type url =
  | File of string
  | Search of string
  | Complex of complex
  | Page_ref of string  (* new added *)
  | Block_ref of string (* new added *)

(* before *)
Inline.Block_reference "id"
(* after *)
Inline.Link {url=inline.Block_ref "id"; ...}

(*
parse "[[a]]"
*)

(*before*)
Link
{ url = Search "a"
; label = [ Plain "" ]
; title = None
; full_text = "[[a]]"
; metadata = ""
}

(*after*)
Link
{ url = Page_ref "a"
; label = [ Plain "" ]
; title = None
; full_text = "[[a]]"
; metadata = ""
}
(*----------------------
parse "[label]([[aaa]])"
*)
(*before*)
Link
{ url = Search "[[page-ref]]"
; label = [ Plain "label" ]
; title = None
; full_text = "[label]([[page-ref]])"
; metadata = ""
}

(*after*)
Link
{ url = Page_ref "page-ref"
; label = [ Plain "label" ]
; title = None
; full_text = "[label]([[page-ref]])"
; metadata = ""}