Add advice to reword .unique :with using :as when possible

The :with has O(n²/2) complexity and basically slows to a crawl with
2000+ element lists. 

my @p = 1..3000; @ = @p.unique: :with(&[eqv]); say now - INIT now; # 25.42728348
my @p = 1..3000; @ = @p.unique: :as(*.Str); say now - INIT now;    # 0.0185597
say 25.427/.0186; #  1367.043011

Author: zoffixznet

doc/Type/List.pod6

@@ -658,6 +658,10 @@ Example:
     my @list = {a => 42}, {b => 13}, {a => 42};
     say @list.unique(:with(&[eqv]))               # OUTPUT: «({a => 42} {b => 13})␤»
 
+B<Note:> since C<:with> L<Callable> has to be tried with all the items in the list,
+this makes C<unique> follow a path with much higher algorithmic complexity. You should
+try to use the C<:as> argument instead, whenever possible.
+
 =head2 routine repeated
 
 Defined as: