Extend memo interface

[parzhitsky]

Blocked by #48, #59, #66 #70

Would be convenient to get sequence start (the very first sequence number), sequence head (list of the first few numbers, see #48), as well as its end (the last item, same as memo[0]) as named properties of memo:

interface Memo {
  /** Sequence's first number (`undefined` for non-iterable ranges) */
  first: number | undefined;

  /** Sequence's latest number (`undefined` for non-iterable ranges) */
  last: number | undefined;

  /** Sequence head (empty for non-iterable ranges) */
  head: number[];
}

Probably, worth adding (though with consideration of #61) No, this won’t be added:

interface Memo {
  /** Sequence tail (empty for non-iterable ranges) */
  // a stack, filled using `memo.tail.push(number)` (while memo itself is filled using `memo.unshift(number)`)
  tail: number[];
}