(position haystack needle (? from))
  haystack: a deque
  needle: any type
  from: an int (optional)
  returns an int or #n

Searches within a deque.

position behaves differently for arrays and strings:

• If haystack is an array, needle must be a callable value. We return the index of the first element in the array for which (needle elem) returns a value other than #n or #f. If there is no such element, we return #n.

• If haystack is a string, needle can be either a character (in which case the index of the first matching character is returned) or a non-empty string (in which case the starting index of the first matching substring is returned).

When from is specified, it must be an integer index, which acts as the starting point for the search.