Add `vec_all_duplicate()`

[DavisVaughan]

PR 2 of 3 for 0.3.0

Adds vec_all_duplicate(), now correctly implemented using dictionary utilities.

The main problem with the one that was previously implemented with equal utilities was that it worked like vec_all_same() ~== vec_all_equal(x, x[1]). This is not what is expected, since we want to detect "all values are duplicated elsewhere", not "all values are the same in this vector".

Practically this means that this now correctly returns TRUE, where with vec_all_same() it would return FALSE:

vctrs::vec_all_duplicate(c(1, 1, 2, 2))
#> [1] TRUE

A follow up PR will change vec_duplicate_any() to vec_any_duplicate()