Reorganize code and document functions

lib/state.mli

@@ -0,0 +1,24 @@
+type t
+(** Log navigation state.
+
+    A [t] represents a loaded log file and a cursor pointing to the currently
+    active line. *)
+
+val create : string -> t
+(** [create file] loads the log file into memory and initializes the cursor at
+    the first line. *)
+
+val show_line : t -> string
+(** [show_line s] returns the line currently pointed to by the cursor. *)
+
+val next : t -> t
+(** [next s] moves the cursor to the next line if possible. *)
+
+val prev : t -> t
+(** [prev s] moves the cursor to the previous line if possible. *)
+
+val cursor : t -> int
+(** Current cursor position. *)
+
+val size : t -> int
+(** Number of lines in the log. *)