normalize length_fun thing_with_length i is just i, unless i is negative, in which case it's length_fun thing_with_length + i. This is used by various python-style slice functions.
val normalize : length_fun:('a -> int) -> 'a -> int -> int
val slice : length_fun:('a -> int) ->
sub_fun:('a -> pos:int -> len:int -> 'a) -> 'a -> int -> int -> 'a
get_pos_len, get_pos_len_exn, and validate_pos_len_exn are intended to be used by functions that take a sequence (array, string, bigstring, ...) and an optional pos and len specifying a subrange of the sequence. Such functions should call get_pos_len with the length of the sequence and the optional pos and len, and it will return the pos and len specifying the range, where the default pos is zero and the default len is to go to the end of the sequence.

It should be the case that:

      pos >= 0 && len >= 0 && pos + len <= length

Note that this allows pos = length and len = 0, i.e. an empty subrange at the end of the sequence.

get_pos_len returns (pos', len') specifying a subrange where:

      pos' = match pos with None -> 0 | Some i -> i
      len' = match len with None -> length - pos | Some i -> i
val get_pos_len : ?pos:int -> ?len:int -> length:int -> (int * int, string) Result.t
val get_pos_len_exn : ?pos:int -> ?len:int -> length:int -> int * int
val check_pos_len_exn : pos:int -> len:int -> length:int -> unit
check_pos_len_exn ~pos ~len ~length raises unless pos >= 0 && len >= 0 && pos + len <= length.