Add _exn variants to functions in In_channel

stdlib/in_channel.mli

@@ -31,16 +31,28 @@ val close_noerr : t -> unit
 
 val input_byte : t -> int option
 
+val input_byte_exn : t -> int
+
 val input_char : t -> char option
 
+val input_char_exn : t -> char
+
 val input_line : t -> string option
 
+val input_line_exn : t -> string
+
 val input_binary_int : t -> int option
 
+val input_binary_int_exn : t -> int
+
 val input : t -> bytes -> int -> int -> int
 
 val really_input : t -> bytes -> int -> int -> unit option
 
+val really_input_exn : t -> bytes -> int -> int -> unit
+
 val really_input_string : t -> int -> string option
 
+val really_input_string_exn : t -> int -> string
+
 val set_binary_mode : t -> bool -> unit