first and last correctly uses affix sizes

[paldepind]

first and last didn't use the stored size of the prefix and suffix when accessing the prefix and suffix. This broke operations that rely on the affix size: prepend and append which mutate the affix and slice which changes the affix sizes to slice.

The change in this PR fixes the bug in first and last. An alternative fix would be to simply define first and last based on nth like this:

function first(l) {
  return nth(0, l);
}

That is simpler but would likely be slightly slower than the specialized first and last implementation. What do you think about those two options @emmanueltouzery?

Fixes #64.

[emmanueltouzery]

so, the new first looks like that:

  const prefixSize = getPrefixSize(l);
  return prefixSize !== 0
    ? l.prefix[prefixSize - 1]
    : l.length !== 0
      ? l.suffix[0]
      : undefined;

and nth:

export function nth<A>(index: number, l: List<A>): A | undefined {
  if (index < 0 || l.length <= index) {
    return undefined;
  }
  const prefixSize = getPrefixSize(l);
  const suffixSize = getSuffixSize(l);
  if (index < prefixSize) {
    return l.prefix[prefixSize - index - 1];
  } else if (index >= l.length - suffixSize) {
    return l.suffix[index - (l.length - suffixSize)];
}

nth doesn't seem much slower for first and last. we could measure it.. but in the end it's your call.

[paldepind]

nth doesn't seem much slower for first and last.

Yes, I agree. I'm going to leave the specialized first and last in place. But, if they need to be changed again I'll likely turn them into nth(0).