FEAT: implement .all_unique()

[samueltardieu]

In day 4 of the Advent of Code 2017 (https://adventofcode.com/),
all_unique() would have been useful. It is already present
in some languages such as Factor's all-unique?.

[bluss]

Hi, I have to say I have had problem seeing the value in such a method, but I have also had trouble coming up with examples for how to do this easily with existing iterator and itertools functionality.

If we have a collection as starting point we can do the following which many users should be able to find:

let all_unique = v.iter().unique().count() == v.len();

(So I realized .unique().count() could be more efficient, and #254 does that.)

I was wondering if we should instead add methods to the unique() and unique_by() iterators, but I'm not sure we end up with the right semantics, or what we do if they are called when the unique() is already partially iterated.

Something like this:

let all_unique = collection.iter().unique().all_unique();

or even a method that tells us about both the numbers. The fidelity loss is that the .unique_count() method can not tell us about the dups that were seen before the .unique_count() method was called (if the .unique() was already partially iterated).

let (count_unique, count_dups) = collection.iter().unique().unique_count();

[jswrenn]

Sorry for the delay!

bors r+

[jswrenn]

bors r+

Edit: Looks like bors doesn't like it when the repository underlying a PR is deleted. :(